Clean up spl_fixedarray.c

Clean up spl_fixedarray.c

Remove inline.
Remove old folding blocks.
Convert an int usage to bool.

Convert some uses of int and size_t into zend_long. This is
incomplete because get_gc requires `int *n`, which should probably
become zend_long or size_t eventually.

Adds spl_fixedarray_empty to help enforce invariants.
Adds spl_fixedarray_default_ctor.
Documents some functions.

Reworks spl_fixedarray_copy into two functions:
  - spl_fixedarray_copy_ctor
  - spl_fixedarray_copy_range

I'm hoping to eventually export SplFixedArray for extensions to
use directly, which is the motivation here.