hex.h: move some hex-related declarations from cache.h

hex.h: move some hex-related declarations from cache.h

hex.c contains code for hex-related functions, but for some reason these
functions were declared in the catch-all cache.h.  Move the function
declarations into a hex.h header instead.

This also allows us to remove includes of cache.h from a few C files.
For now, we make cache.h include hex.h, so that it is easier to review
the direct changes being made by this patch.  In the next patch, we will
remove that, and add the necessary direct '#include "hex.h"' in the
hundreds of C files that need it.

Note that reviewing the header changes in this commit might be
simplified via
    git log --no-walk -p --color-moved $COMMIT -- '*.h'`
In particular, it highlights the simple movement of code in .h files
rather nicely.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>