Cleanup redundant AsFixedBytes and FixedLength traits (#3131)

* no need for AsFixedBytes we can just use AsRef<[u8]>

* cleanup FixedLength trait

* revert this change for now

* fix store tests

* cleanup and fix tests after rebase

* fix tests

* update based on PR review
less hard-coded values now

* cleanup

p2p/src/conn.rs

@@ -21,7 +21,7 @@
 //! stream and make sure we get the right number of bytes out.
 
 use crate::core::ser;
-use crate::core::ser::{FixedLength, ProtocolVersion};
+use crate::core::ser::ProtocolVersion;
 use crate::msg::{
 	read_body, read_discard, read_header, read_item, write_message, Msg, MsgHeader,
 	MsgHeaderWrapper,

p2p/src/msg.rs

@@ -19,7 +19,7 @@ use crate::core::core::hash::Hash;
 use crate::core::core::BlockHeader;
 use crate::core::pow::Difficulty;
 use crate::core::ser::{
-	self, FixedLength, ProtocolVersion, Readable, Reader, StreamingReader, Writeable, Writer,
+	self, ProtocolVersion, Readable, Reader, StreamingReader, Writeable, Writer,
 };
 use crate::core::{consensus, global};
 use crate::types::{
@@ -262,6 +262,9 @@ pub struct MsgHeader {
 }
 
 impl MsgHeader {
+	// 2 magic bytes + 1 type byte + 8 bytes (msg_len)
+	pub const LEN: usize = 2 + 1 + 8;
+
 	/// Creates a new message header.
 	pub fn new(msg_type: Type, len: u64) -> MsgHeader {
 		MsgHeader {
@@ -272,11 +275,6 @@ impl MsgHeader {
 	}
 }
 
-impl FixedLength for MsgHeader {
-	// 2 magic bytes + 1 type byte + 8 bytes (msg_len)
-	const LEN: usize = 2 + 1 + 8;
-}
-
 impl Writeable for MsgHeader {
 	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ser::Error> {
 		ser_multiwrite!(