analyze: track reasons why functions are not rewritten (#1072)

This branch replaces most of the existing `fns_failed`/`fns_fixed`
machinery with a new strategy for marking items with a "don't rewrite"
flag. This makes it easier to integrate new, stricter checks for
unsupported code patterns, with the goal of reducing compile errors in
the rewritten code. Some of these checks are also implemented in this
branch, such as a check for complex (and currently unsupported) `Cell`
rewrites.

When an item is marked as don't-rewrite, we also record a reason flag.
This is helpful for debugging, and will also make it easy in the future
to disable specific checks (by ignoring the corresponding reason flag)
for debugging/testing purposes or by user request.