consensus_encoding: introduce tuple encoders

diff --git a/consensus_encoding/Cargo.toml b/consensus_encoding/Cargo.toml
index 3951898b..899631d3 100644
--- a/consensus_encoding/Cargo.toml
+++ b/consensus_encoding/Cargo.toml
@@ -132,7 +132,7 @@ stable_sort_primitive = "warn"
 str_split_at_newline = "warn"
 string_add_assign = "warn"
 struct_excessive_bools = "warn"
-struct_field_names = "warn"
+struct_field_names = "allow" # dumb
 too_many_lines = "warn"
 transmute_ptr_to_ptr = "warn"
 trivially_copy_pass_by_ref = "warn"
diff --git a/consensus_encoding/src/encode/encoders.rs b/consensus_encoding/src/encode/encoders.rs
index d4ebb147..d1ec7b32 100644
--- a/consensus_encoding/src/encode/encoders.rs
+++ b/consensus_encoding/src/encode/encoders.rs
@@ -55,3 +55,99 @@ impl<'e, const N: usize> Encoder<'e> for ArrayEncoder<N> {
         false
     }
 }
+
+/// An encoder which encodes two objects, one after the other.
+pub struct Encoder2<A, B> {
+    enc_idx: usize,
+    enc_1: A,
+    enc_2: B,
+}
+
+impl<A, B> Encoder2<A, B> {
+    /// Constructs a new composite encoder.
+    pub fn new(enc_1: A, enc_2: B) -> Self { Self { enc_idx: 0, enc_1, enc_2 } }
+}
+
+impl<'e, A: Encoder<'e>, B: Encoder<'e>> Encoder<'e> for Encoder2<A, B> {
+    fn current_chunk(&self) -> Option<&[u8]> {
+        if self.enc_idx == 0 {
+            self.enc_1.current_chunk()
+        } else {
+            self.enc_2.current_chunk()
+        }
+    }
+
+    fn advance(&mut self) -> bool {
+        if self.enc_idx == 0 {
+            if !self.enc_1.advance() {
+                self.enc_idx += 1;
+            }
+            true
+        } else {
+            self.enc_2.advance()
+        }
+    }
+}
+
+// For now we implement every higher encoder by composing Encoder2s, because
+// I'm lazy and this is trivial both to write and to review. For efficiency, we
+// should eventually unroll all of these. There are only a couple of them. The
+// unrolled versions should be macro-izable, if we want to do that.
+
+/// An encoder which encodes three objects, one after the other.
+pub struct Encoder3<A, B, C> {
+    inner: Encoder2<Encoder2<A, B>, C>,
+}
+
+impl<A, B, C> Encoder3<A, B, C> {
+    /// Constructs a new composite encoder.
+    pub fn new(enc_1: A, enc_2: B, enc_3: C) -> Self {
+        Self { inner: Encoder2::new(Encoder2::new(enc_1, enc_2), enc_3) }
+    }
+}
+
+impl<'e, A: Encoder<'e>, B: Encoder<'e>, C: Encoder<'e>> Encoder<'e> for Encoder3<A, B, C> {
+    fn current_chunk(&self) -> Option<&[u8]> { self.inner.current_chunk() }
+    fn advance(&mut self) -> bool { self.inner.advance() }
+}
+
+/// An encoder which encodes four objects, one after the other.
+pub struct Encoder4<A, B, C, D> {
+    inner: Encoder2<Encoder2<A, B>, Encoder2<C, D>>,
+}
+
+impl<A, B, C, D> Encoder4<A, B, C, D> {
+    /// Constructs a new composite encoder.
+    pub fn new(enc_1: A, enc_2: B, enc_3: C, enc_4: D) -> Self {
+        Self { inner: Encoder2::new(Encoder2::new(enc_1, enc_2), Encoder2::new(enc_3, enc_4)) }
+    }
+}
+
+impl<'e, A: Encoder<'e>, B: Encoder<'e>, C: Encoder<'e>, D: Encoder<'e>> Encoder<'e> for Encoder4<A, B, C, D> {
+    fn current_chunk(&self) -> Option<&[u8]> { self.inner.current_chunk() }
+    fn advance(&mut self) -> bool { self.inner.advance() }
+}
+
+/// An encoder which encodes six objects, one after the other.
+pub struct Encoder6<A, B, C, D, E, F> {
+    inner: Encoder2<Encoder3<A, B, C>, Encoder3<D, E, F>>,
+}
+
+impl<A, B, C, D, E, F> Encoder6<A, B, C, D, E, F> {
+    /// Constructs a new composite encoder.
+    pub fn new(enc_1: A, enc_2: B, enc_3: C, enc_4: D, enc_5: E, enc_6: F) -> Self {
+        Self {
+            inner: Encoder2::new(
+                Encoder3::new(enc_1, enc_2, enc_3),
+                Encoder3::new(enc_4, enc_5, enc_6),
+            ),
+        }
+    }
+}
+
+impl<'e, A: Encoder<'e>, B: Encoder<'e>, C: Encoder<'e>, D: Encoder<'e>, E: Encoder<'e>, F: Encoder<'e>> Encoder<'e>
+    for Encoder6<A, B, C, D, E, F>
+{
+    fn current_chunk(&self) -> Option<&[u8]> { self.inner.current_chunk() }
+    fn advance(&mut self) -> bool { self.inner.advance() }
+}
diff --git a/consensus_encoding/src/lib.rs b/consensus_encoding/src/lib.rs
index 17aee65b..8f7e7551 100644
--- a/consensus_encoding/src/lib.rs
+++ b/consensus_encoding/src/lib.rs
@@ -16,5 +16,7 @@
 
 mod encode;
 
-pub use self::encode::encoders::{ArrayEncoder, BytesEncoder};
+pub use self::encode::encoders::{
+    ArrayEncoder, BytesEncoder, Encoder2, Encoder3, Encoder4, Encoder6,
+};
 pub use self::encode::{Encodable, Encoder};
diff --git a/primitives/Cargo.toml b/primitives/Cargo.toml
index f273e6d1..4709c975 100644
--- a/primitives/Cargo.toml
+++ b/primitives/Cargo.toml
@@ -145,7 +145,7 @@ stable_sort_primitive = "warn"
 str_split_at_newline = "warn"
 string_add_assign = "warn"
 struct_excessive_bools = "warn"
-struct_field_names = "allow" # TODO: Triggers warning for `witness_elements`.
+struct_field_names = "allow" # dumb
 too_many_lines = "warn"
 transmute_ptr_to_ptr = "warn"
 trivially_copy_pass_by_ref = "warn"
diff --git a/units/Cargo.toml b/units/Cargo.toml
index 64a2877a..428c6bf3 100644
--- a/units/Cargo.toml
+++ b/units/Cargo.toml
@@ -143,7 +143,7 @@ stable_sort_primitive = "warn"
 str_split_at_newline = "warn"
 string_add_assign = "warn"
 struct_excessive_bools = "warn"
-struct_field_names = "warn"
+struct_field_names = "allow" # dumb
 too_many_lines = "warn"
 transmute_ptr_to_ptr = "warn"
 trivially_copy_pass_by_ref = "warn"