Make `toBuilder` and `formBuiler` APIs more unified

[4eUeP]

For now, if we want to build a Builder from bytes, int ... we can find these functions under Z.Data.Builder, but the CBytes's toBuilder is under Z.Data.CBytes.

What about putting these builder functions in one place? @winterland1989

[winterland1989]

CBytes is a more independent module, so I didn’t want to make a reversed dependency. And if we move them to Builder then we have to explain why it may write invalid UTF8 bytes, which is somewhat duplicated.