Revert ObligationForest on beta branch

[nikomatsakis]

The OF caused a regression in compiler performance and backporting the fix
to beta was deemed too risky and too challenging, since it is
non-trivial and builds on prior commits in various ways.

[rust-highfive]

r? @arielb1

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[rust-highfive]

Warning

• Pull requests are usually filed against the master branch for this repo, but this one is against beta. Please double check that you specified the right target!

[nikomatsakis]

Hmm, local testing of this PR has encountered some odd failures (e.g., a segfault just now in the libcollections tests).

But I am sort of reluctant to blame it on this PR to be honest.

[brson]

I'm testing locally.

[nikomatsakis]

Actually, I see the same problem whether or not this commit is applied:

test str::test_into_bytes ... ok
test str::test_empty_match_indices ... ok
test str::test_find_str ... ok
Illegal instruction (core dumped)
make: *** [tmp/check-stage2-T-x86_64-unknown-linux-gnu-H-x86_64-unknown-linux-gnu-collectionstest.ok] Error 132

[brson]

I don't see the crash locally.

[nikomatsakis]

OK, I don't know what's happening locally, but I'm inclined to take this PR.