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/state/notification_event.go

@@ -39,9 +39,9 @@ func (ne *NotificationEvent) DecodeBinary(r *io.BinReader) {
 // EncodeBinary implements the Serializable interface.
 func (aer *AppExecResult) EncodeBinary(w *io.BinWriter) {
 	w.WriteBytes(aer.TxHash[:])
-	w.WriteLE(aer.Trigger)
+	w.WriteByte(aer.Trigger)
 	w.WriteString(aer.VMState)
-	w.WriteLE(aer.GasConsumed)
+	aer.GasConsumed.EncodeBinary(w)
 	w.WriteString(aer.Stack)
 	w.WriteArray(aer.Events)
 }
@@ -49,9 +49,9 @@ func (aer *AppExecResult) EncodeBinary(w *io.BinWriter) {
 // DecodeBinary implements the Serializable interface.
 func (aer *AppExecResult) DecodeBinary(r *io.BinReader) {
 	r.ReadBytes(aer.TxHash[:])
-	r.ReadLE(&aer.Trigger)
+	aer.Trigger = r.ReadByte()
 	aer.VMState = r.ReadString()
-	r.ReadLE(&aer.GasConsumed)
+	aer.GasConsumed.DecodeBinary(r)
 	aer.Stack = r.ReadString()
 	r.ReadArray(&aer.Events)
 }