liblzma: Add lzma_memcmplen() for fast memory comparison.

This commit just adds the function. Its uses will be in
separate commits.

This hasn't been tested much yet and it's perhaps a bit early
to commit it but if there are bugs they should get found quite
quickly.

Thanks to Jun I Jin from Intel for help and for pointing out
that string comparison needs to be optimized in liblzma.
liblzma: Use lzma_memcmplen() in the match finders.

This doesn't change the match finder output.
Update THANKS.
liblzma: Use lzma_memcmplen() in fast mode of LZMA.
liblzma: Simplify LZMA fast mode code by using memcmp().
liblzma: Use lzma_memcmplen() in normal mode of LZMA.

Two locations were not changed yet because the simplest change
assumes that the initial "len" may be greater than "limit".