libsecp256k1

test: Add option to display all available tests

Useful option to avoid opening the large tests.c file just to find
the test case you want to run.

test: support running specific tests/modules targets

Add support for specifying single tests or modules to run via the
"--target" or "-t" command-line option. Multiple targets can be
provided; only the specified tests or all tests in the specified
module/s will run instead of the full suite.

Examples:
-t=<test name> runs an specific test.
-t=<module name> runs all tests within the specified module.

Both options can be provided multiple times.

test: add --help for command-line options

Add a help message for the test suite, documenting available options,
defaults, and backward-compatible positional arguments.

test: introduce (mini) unit test framework

Lightweight unit testing framework, providing a structured way to define,
execute, and report tests. It includes a central test registry, a flexible
command-line argument parser of the form "--key=value" / "-k=value" /
"-key=value" (facilitating future framework extensions), ability to run
tests in parallel and accumulated test time logging reports.

So far the supported command-line args are:
- "--jobs=<num>" or "-j=<num>" to specify the number of parallel workers.
- "--seed=<hex>" to specify the RNG seed (random if not set).
- "--iterations=<num>" or "-i=<num>" to specify the number of iterations.

Compatibility Note:
To stay compatible with previous versions, the framework also supports
the two original positional arguments: the iterations count and the
RNG seed (in that order).

test: adapt modules to the new test infrastructure

This not only provides a structural improvement but also
allows us to (1) specify individual tests to run and (2)
execute each of them concurrently.

docs: Improve API docs of _context_set_illegal_callback

docs: Clarify that callback can be called more than once

build: Fix warnings in x86_64 assembly check

This change fixes:
- `-Wuninitialized` in both Autotools and CMake;
- `-Wreturn-type` in CMake only.

bench: improve context creation in ECDH benchmark

Calling `secp256k1_context_create` with `SECP256K1_FLAGS_TYPE_CONTEXT`
seems to be not strictly API-compliant, as the only allowed
(non-deprecated) value is `SECP256K1_CONTEXT_NONE`, even if the
former happens to map to the latter currently.

Fix this by not dynamically creating a context in the first place and
switch to using the static context, as it is sufficient for this
benchmark and presumably matches what the "no capabilities" comment
intended back then.

doc: clarify API doc of `secp256k1_ecdsa_recover` return value

Co-authored-by: Tim Ruffing <me@real-or-random.org>

refactor: move 'gettime_i64()' to tests_common.h

Relocate the clock time getter to tests_common.h to
make it easily reusable across test programs. This
will be useful for the upcoming unit test framework.

Context - why not placing it inside testutil.h?:
The bench program links against the production-compiled library,
not its own compiled version. Therefore, `gettime_i64()` cannot
be moved to testutil.h, because testutil.h calls
`secp256k1_pubkey_save()`, which exists only in the internal
secp256k1.c and not in the public API.

test: introduce group order byte-array constant for deduplication

Split memclear into two versions

secp256k1_memclear has the side effect of undefining bytes for
valgrind checks. In some cases, we may want to zero bytes
but allow subsequent reads. So we split memclear into
memclear_explicit, which makes no guarantees about the content
of the buffer on return, and memzero_explicit, which guarantees
zero value on return.

Change the memset in partial_sign to use memzero_explicit.

check-abi: remove support for obsolete CMake library output location (src/libsecp256k1.so)

The CMake library output location was changed from "src/" to "lib/"
in PR #1553, supporting the old location shouldn't be necessary anymore.

doc: mention ctx requirement for `_ellswift_create` (not secp256k1_context_static)

Rename and clear var containing k or -k

buf currently holds k or -k and isn't cleared, so clear it and rename to
nonce32 to clarify its sensitivity and match how it is named in the
corresponding ECDSA sign_inner.

Use size_t instead of int for RFC6979 outlen copy

If outlen is > INT_MAX, could trigger segfault or hang after copy
int now = outlen.

ci: Add more tests for clang-cl

doc: Recommend clang-cl when building on Windows

tests: refactor tagged hash tests

Move the sha256_tag_test_internal function out of the musig module
into tests.c. This makes it available to other modules wishing to verify tagged
hashes without needing to duplicate the function.

Change the function signature to expect a const unsigned char and update
the tagged hash tests to use static const unsigned char character
arrays (where necessary).

Add a comment for each tag. This is done as a convenience for checking
the strings against the protocol specifications, where the tags are
normally specified as strings.

Update tests in the ellswift and schnorrsig modules to use the
sha256_tag_test_internal helper function.

docs: fix broken link to eprint cache.pdf paper

doc: Add `--gcov-ignore-parse-errors=all` option to `gcovr` invocations

Otherwise, commands might fail due to bugs in the `gcov` tool.

doc: Add `--merge-mode-functions=separate` option to `gcovr` invocations

Otherwise, commands fail with the error:
```
Stderr of gcov was >><< End of stderr
Exception was >>Got function secp256k1_scalar_split_lambda on multiple lines: 67, 142.
You can run gcovr with --merge-mode-functions=MERGE_MODE.
The available values for MERGE_MODE are described in the documentation.<< End of stderr
```

doc: Exclude modules' `bench_impl.h` headers from coverage report

autotools, docs: Adjust help string for `--enable-coverage` option