Add available_formats(), available_subtypes() and format_check()

The named constants should be moved as `enum`s into the `cdef` at a later point ...

@bastibe: you had some reservations about these functions:
https://github.com/mgeier/PySoundFile/commit/88ab0efd639b64c5e21baa553676fdc2dc9946b3#comments

Please comment if you still have.