Revert "Deprecate simd in libcore. Use libsimd instead."

This reverts commit bc44813b970bc3ea9d626d57156ca3bcaab29764.

Conflicts:
	src/libsimd/lib.rs
Revert part of "libsimd: Plaster experimental attributes everywhere. Also fixed a test."

This partially reverts commit 5a1a1616a76f8a8bc6137880b2fbd7fbd42bac1a.

Conflicts:
	src/libsimd/lib.rs
compiletest: Renovate PNaCl/NaCl's build process.