gen: fix decoding for non-[]byte types when type-conversion needed

For example for the annotation below:
```
//msgp:shim uuid.UUID as:[]byte using:uuidToBytes/bytesToUUID
```
When an UUID is an array of bytes (and not a slice of bytes).
We need to use `uuidToBytes()` call and not hardcoded `[]byte()` cast from the generated func `DecodeMsg(dc *msgp.Reader) error`

Ex:
```
func (z *Foobar) DecodeMsg(dc *msgp.Reader) (err error) {
	[...]
	case "Id":
	{
		var zb0002 []byte
-		zb0002, err = dc.ReadBytes([]byte(z.Id))
+		zb0002, err = dc.ReadBytes(uuidToBytes(z.Id))
```

gen/decode.go

@@ -155,7 +155,8 @@ func (d *decodeGen) gBase(b *BaseElem) {
 	switch b.Value {
 	case Bytes:
 		if b.Convert {
-			d.p.printf("\n%s, err = dc.ReadBytes([]byte(%s))", tmp, vname)
+			lowered := b.ToBase() + "(" + vname + ")"
+			d.p.printf("\n%s, err = dc.ReadBytes(%s)", tmp, lowered)
 		} else {
 			d.p.printf("\n%s, err = dc.ReadBytes(%s)", vname, vname)
 		}