Make math types subclasses of Data.

Should've done this early on, but yeah, I didn't.
Version 1.5.0

- Makes all math types subclasses of the Data class. This is mainly for
  compatibility with another gem of mine (snow-data), but also because
  it's more or less the proper thing to do.
Add #bytesize as an alias of #size on all math types.

Another product of snow-data compatibility.
Version 1.5.1

- Aliases #size as #bytesize for all math types.