Fix test_load.rb

This was recently enabled in 4b6dfbc, probably accidentally.  Happy
accident though: it's working in 1.9 now once we re-add the
requireTest-1.0.jar, so that's one less FIXME.
Remove failing ffi-dependent test

With ffi being removed in 1ca75b2, this test is no longer valid.
Improve String#rindex performance for pathological cases

When the haystack string contained a non-single-byte character, code
would previously call `StringSupport.nth` for every backwards
step. `nth` walks all the way through the string from the
beginning. The combination of these caused O(n^2) behavior.

Now `rindex` follows MRI's pattern of simply walking backward until we
find the previous character.

Fixes #1140
Merging with 1.7 branch
[IR] Collapse 1.8/1.9/2.0 IR builders into Ruby 2.0 builder.

* Got rid of 1.8 and 1.9 only support in the IR builder.
Simple merge conflict
Hmm resync github repo