primitives: Add Default and new to all decoders

diff --git a/primitives/src/block.rs b/primitives/src/block.rs
index bedbc40b..bf9f9f88 100644
--- a/primitives/src/block.rs
+++ b/primitives/src/block.rs
@@ -14,9 +14,9 @@ use core::marker::PhantomData;
 
 #[cfg(feature = "arbitrary")]
 use arbitrary::{Arbitrary, Unstructured};
+use encoding::{ArrayDecoder, Decodable, Decoder, Decoder6, Encodable};
 #[cfg(feature = "alloc")]
 use encoding::{CompactSizeEncoder, Decoder2, Encoder2, SliceEncoder, VecDecoder};
-use encoding::{Decodable, Decoder, Decoder6, Encodable};
 use hashes::{sha256d, HashEngine as _};
 use internals::write_err;
 
@@ -373,6 +373,17 @@ type BlockInnerDecoder = Decoder2<HeaderDecoder, VecDecoder<Transaction>>;
 #[cfg(feature = "alloc")]
 pub struct BlockDecoder(BlockInnerDecoder);
 
+#[cfg(feature = "alloc")]
+impl BlockDecoder {
+    /// Constructs a new [`Block`] decoder.
+    pub const fn new() -> Self { Self(Decoder2::new(HeaderDecoder::new(), VecDecoder::new())) }
+}
+
+#[cfg(feature = "alloc")]
+impl Default for BlockDecoder {
+    fn default() -> Self { Self::new() }
+}
+
 #[cfg(feature = "alloc")]
 impl Decoder for BlockDecoder {
     type Output = Block;
@@ -673,6 +684,18 @@ type HeaderInnerDecoder = Decoder6<
 pub struct HeaderDecoder(HeaderInnerDecoder);
 
 impl HeaderDecoder {
+    /// Constructs a new [`Header`] decoder.
+    pub const fn new() -> Self {
+        Self(Decoder6::new(
+            VersionDecoder::new(),
+            BlockHashDecoder::new(),
+            TxMerkleNodeDecoder::new(),
+            BlockTimeDecoder::new(),
+            CompactTargetDecoder::new(),
+            ArrayDecoder::new(),
+        ))
+    }
+
     fn from_inner(e: <HeaderInnerDecoder as Decoder>::Error) -> HeaderDecoderError {
         match e {
             encoding::Decoder6Error::First(e) => HeaderDecoderError::Version(e),
@@ -685,6 +708,10 @@ impl HeaderDecoder {
     }
 }
 
+impl Default for HeaderDecoder {
+    fn default() -> Self { Self::new() }
+}
+
 impl Decoder for HeaderDecoder {
     type Output = Header;
     type Error = HeaderDecoderError;
diff --git a/primitives/src/transaction.rs b/primitives/src/transaction.rs
index 450c5344..82efde1f 100644
--- a/primitives/src/transaction.rs
+++ b/primitives/src/transaction.rs
@@ -971,6 +971,23 @@ type TxInInnerDecoder = Decoder3<OutPointDecoder, ScriptSigBufDecoder, SequenceD
 #[cfg(feature = "alloc")]
 pub struct TxInDecoder(TxInInnerDecoder);
 
+#[cfg(feature = "alloc")]
+impl TxInDecoder {
+    /// Constructs a new [`TxIn`] decoder.
+    pub const fn new() -> Self {
+        Self(Decoder3::new(
+            OutPointDecoder::new(),
+            ScriptSigBufDecoder::new(),
+            SequenceDecoder::new(),
+        ))
+    }
+}
+
+#[cfg(feature = "alloc")]
+impl Default for TxInDecoder {
+    fn default() -> Self { Self::new() }
+}
+
 #[cfg(feature = "alloc")]
 impl Decoder for TxInDecoder {
     type Output = TxIn;
@@ -1071,6 +1088,19 @@ type TxOutInnerDecoder = Decoder2<AmountDecoder, ScriptPubKeyBufDecoder>;
 #[cfg(feature = "alloc")]
 pub struct TxOutDecoder(TxOutInnerDecoder);
 
+#[cfg(feature = "alloc")]
+impl TxOutDecoder {
+    /// Constructs a new [`TxOut`] decoder.
+    pub const fn new() -> Self {
+        Self(Decoder2::new(AmountDecoder::new(), ScriptPubKeyBufDecoder::new()))
+    }
+}
+
+#[cfg(feature = "alloc")]
+impl Default for TxOutDecoder {
+    fn default() -> Self { Self::new() }
+}
+
 #[cfg(feature = "alloc")]
 impl Decoder for TxOutDecoder {
     type Output = TxOut;