key_def: make key_compare take keys without header

We need to compare a tuple position with a search key in select() and
pairs() to make sure the tuple position meets the search criteria. The
problem is that we strip the MessagePack header from the position while
key_compare() takes keys with headers. Let's make key_compare take keys
without headers like the rest of comparator functions. Since in Vinyl we
often need to compare keys with headers, we also add vy_key_compare()
helper function.

Needed for #8403
Needed for #8404

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring