resampler: Make resampler struct implementation agnostic.

The pa_resampler struct contains many implementation specific
structures. These create overhead and don't belong there anyways.

This patch moves the implementation specific structures out of the
pa_resampler structure.
resampler: Move the fix method logic into a separate function
resampler: fix_method use switch instead of if/else.

This way the fix method function should be more readable and easier to
expand in the future.
resampler: Move the work format finding logic into a separate function.
resampler: choose_work_format use switch instead of if/else

This way the function for choosing the work format should
be more readable and easier to expand in the future.