Fletcher4: save/reload implementation context

Init, compute, and fini methods are changed to work on internal context object.
This is necessary because ABI does not guarantee that SIMD registers will be preserved
on function calls. This is technically the case in Linux kernel in between
kfpu_begin()/kfpu_end(), but it breaks user-space tests and some kernels that
don't require disabling preemption for using SIMD (osx).

Use scalar compute methods in-place for small buffers, and when the buffer size
does not meet SIMD size alignment.

Signed-off-by: Gvozden Neskovic <neskovic@gmail.com>