Fixed extrinsic encoding (#794)

* Fixed extrinsic encoding

* Reserve heuristic

* Fixed no-std build

substrate/core/sr-primitives/src/generic/mod.rs

@@ -35,3 +35,28 @@ pub use self::checked_extrinsic::CheckedExtrinsic;
 pub use self::header::Header;
 pub use self::block::{Block, SignedBlock, BlockId};
 pub use self::digest::{Digest, DigestItem, DigestItemRef};
+
+use codec::Encode;
+use rstd::prelude::*;
+
+fn encode_with_vec_prefix<T: Encode, F: Fn(&mut Vec<u8>)>(encoder: F) -> Vec<u8> {
+	let size = ::rstd::mem::size_of::<T>();
+	let reserve = match size {
+		0...0b00111111 => 1,
+		0...0b00111111_11111111 => 2,
+		_ => 4,
+	};
+	let mut v = Vec::with_capacity(reserve + size);
+	v.resize(reserve, 0);
+	encoder(&mut v);
+
+	// need to prefix with the total length to ensure it's binary comptible with
+	// Vec<u8>.
+	let mut length: Vec<()> = Vec::new();
+	length.resize(v.len() - reserve, ());
+	length.using_encoded(|s| {
+		v.splice(0..reserve, s.iter().cloned());
+	});
+
+	v
+}