chain_getBlock extrinsics encoding (#1024)

* deserialize without decoding

* change decoding approach

* fix tests

* decode without allocating

* strip compact prefix

* cargo fmt

* nit adjustment

subxt/src/utils/mod.rs

@@ -11,7 +11,7 @@ mod multi_signature;
 mod static_type;
 mod wrapper_opaque;
 
-use codec::{Decode, Encode};
+use codec::{Compact, Decode, Encode};
 use derivative::Derivative;
 
 pub use account_id::AccountId32;
@@ -35,6 +35,14 @@ impl codec::Encode for Encoded {
     }
 }
 
+/// Decodes a compact encoded value from the beginning of the provided bytes,
+/// returning the value and any remaining bytes.
+pub(crate) fn strip_compact_prefix(bytes: &[u8]) -> Result<(u64, &[u8]), codec::Error> {
+    let cursor = &mut &*bytes;
+    let val = <Compact<u64>>::decode(cursor)?;
+    Ok((val.0, *cursor))
+}
+
 /// A version of [`std::marker::PhantomData`] that is also Send and Sync (which is fine
 /// because regardless of the generic param, it is always possible to Send + Sync this
 /// 0 size type).