reg: rename Bytes() to Size() (#74)

It was pointed out #73 that Bytes() is a poor name for the size of the register in bytes. In idiomatic Go you would probably expect a Bytes() method to return []byte.

This diff changes the Bytes() to Size(). As a result the Size type also needed to be renamed, and Width seemed a reasonable choice.

Fixes #73

operand/checks.go

@@ -132,7 +132,7 @@ func IsYMM(op Op) bool {
 // IsRegisterKindSize returns true if op is a register of the given kind and size in bytes.
 func IsRegisterKindSize(op Op, k reg.Kind, n uint) bool {
 	r, ok := op.(reg.Register)
-	return ok && r.Kind() == k && r.Bytes() == n
+	return ok && r.Kind() == k && r.Size() == n
 }
 
 // IsRegisterKind returns true if op is a register of the given kind.