add mismatch_complexity

add mismatch_complexity

The mismatch_complexity variable allows us to only print out the most
complex kind of mismatch, and skip over simpler mismatches, in order to
keep the mismatch reporting as simple to understand as possible.

When assert_resolvability is false, mismatch_complexity simply remembers
the most complex mismatch we've met so far during each pass (e.g.
unresolved() is 0, mismatch(e) is 1, mismatch(tok, t1, t2) is 2, and
mismatch(tok, t, decl, index) is 3).

Once we turn on assert_resolvability, we check mismatch_complexity in
mismatch() and unresolved() to determine whether to print out an error
or not. If mismatch_complexity is higher than our own complexity, then
we don't print anything since we know that there is a better-suited
mismatch coming up.