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/smartcontract/param_context.go

@@ -80,12 +80,12 @@ func (pt ParamType) MarshalJSON() ([]byte, error) {
 
 // EncodeBinary implements io.Serializable interface.
 func (pt ParamType) EncodeBinary(w *io.BinWriter) {
-	w.WriteBytes([]byte{byte(pt)})
+	w.WriteByte(byte(pt))
 }
 
 // DecodeBinary implements io.Serializable interface.
 func (pt *ParamType) DecodeBinary(r *io.BinReader) {
-	r.ReadLE(pt)
+	*pt = ParamType(r.ReadByte())
 }
 
 // NewParameter returns a Parameter with proper initialized Value