Remove redundant a_generic kernels

[argilo]

Also, rename u_generic to generic.

After this change, every kernel has exactly one generic protokernel, except for volk_32u_reverse_32u, which never had a generic protokernel.

Update: volk_32u_reverse_32u now has a generic protokernel as well. See below for details.

Fixes #516.

[argilo]

The test failure is unrelated; it's #676.

[argilo]

This change will also help with #516 (since more kernels will have generic), but it's still not fully resolved until either:

• every kernel has generic and we enforce that in volk_kernel_defs.py; or
• code that depends on generic existing is removed.

[argilo]

I asked about this in the VOLK Matrix channel and didn't hear any objections to the first option. So I renamed volk_32u_reverse_32u_dword_shuffle to volk_32u_reverse_32u_generic, giving every kernel a generic implementation, and added an enforcement mechanism to volk_kernel_defs.py.

[jdemel]

LGTM. Also, this fixes #516 .