replace _splat.get_sample_precision() with constant

Remove _splat.get_sample_precision() function and add
_splat.SAMPLE_PRECISION integer value, also imported
in splat.SAMPLE_PRECISION.

+ update tests and usage in splat.data
fix sample format definitions

* add _splat.SAMPLE_INT and _splat.SAMPLE_FLOAT to identify
  whether sound samples are integer or floating point

* rename _splat.Fragment.as_bytes -> export_bytes

* update arguments of _splat.Fragment.{import|export}_bytes to
  use the new sample definitions + add keyword arguments

* update docstrings in splat.data.Fragment

* sample_width is now defined in terms of bits per sample per
  channel instead of a number of bytes

* support only 64-bit float and 16-bit integer for now

* splat/__init__.py: from _splat import SAMPLE_INT, SAMPLE_FLOAT,
  NATIVE_SAMPLE_TYPE, NATIVE_SAMPLE_WIDTH
doc: add section about audio formats