rewriteSharedTermConvertibility: rewrite terms without breaking type safety

[jn80842]

rewriteSharedTermConvertibility should be a replacement for rewriteSharedTerm, but in such a way that dependently typed terms can only be rewritten using convertible rewrite rules.

[brianhuffman]

We don't need to have both rewriteSharedTermTypeSafe and rewriteSharedTermConvertibility; your new version here should just replace rewriteSharedTermTypeSafe, as it's intended to be an improvement to it. Removing the original function will also make the diff easier to read, as it will highlight what you've changed vs what you've copied from the original.

[brianhuffman]

There's a single integration test failure: test_hash_table is timing out. I might be able to help debug that.

[brianhuffman]

While preparing #2684 I ran into a very similar rewriter-related timeout in test_hash_table. That problem was fixed in 32b55dc. I expect this one has a similar cause, namely that a term isn't getting simplified enough.