io: add type-specific read/write methods

This seriously improves the serialization/deserialization performance for
several reasons:
 * no time spent in `binary` reflection
 * no memory allocations being made on every read/write
 * uses fast ReadBytes everywhere it's appropriate

It also makes Fixed8 Serializable just for convenience.

pkg/core/transaction/miner.go

@@ -12,10 +12,10 @@ type MinerTX struct {
 
 // DecodeBinary implements Serializable interface.
 func (tx *MinerTX) DecodeBinary(r *io.BinReader) {
-	r.ReadLE(&tx.Nonce)
+	tx.Nonce = r.ReadU32LE()
 }
 
 // EncodeBinary implements Serializable interface.
 func (tx *MinerTX) EncodeBinary(w *io.BinWriter) {
-	w.WriteLE(tx.Nonce)
+	w.WriteU32LE(tx.Nonce)
 }