refs: Speed up packed lookups.

Currently ref lookups require loading the entire packed-refs file into
a hashmap in memory. For repos with large numbers of refs this can be
painfully slow.

This patch replaces the existing lookup code and instead mmap()'s the
packed-refs file and performs a binary search to locate the ref entry.
Git uses a similiar approach.

The old hash table codepath is still used for unsorted packed-refs files.

This patch also fixes a minor bug where the "peeled" trait is never
parsed correctly from the packed-refs header.

Threading can now be disabled with `USE_THREADS=OFF` instead of
`THREADSAFE=OFF` to better support the other cmake semantics.

Nanosecond support is the default _if_ we can detect it.  This should be
our default always - like threads - and people can opt out explicitly.

Commit 4ae41f9c639d246d34dac89c3f1d9451c9cfa8d3 made `git_odb`
race-free, and added internal locking. Update `docs/threading.md`
accordingly, so that APIs built atop libgit2 (e.g. language bindings)
can count on this.

in #6083 the test runner was renamed to libgit2_tests,
but not all references to the old name were updated.
this change changes all of them to use the new name.