std::str: Bench test for char iterators
std::str: Small fix for slice
std::str: Special case char_range_at_reverse so it is faster

Implement char_range_at_reverse similarly to char_range_at, instead of
re-using that method.
std::str: Add str::raw::slice_unchecked

Add a function like raw::slice_bytes, but it doesn't check slice
boundaries. For iterator use where we always know the begin, end indices
are in range.
std::str: Implement CharIterator separately

Let CharIterator be a separate type from CharOffsetIterator (so that
CharIterator can be cloned, for example).

Implement CharOffsetIterator by using the same technique as the method
subslice_offset.
std::str: Use CharOffsetIterator in .find() and .rfind()
std::str: Correct docstrings for lack of null terminator in ~str and &str
std::str: Only check char boundary for end index in .slice_to()
std::str: Use CharOffsetIterator in slice_chars
std::str: Improve comments for CharIterator
Fix some parsing errors in extra::url

Fixes issue #8612.
std::str: Use iterators instead of while loops for CharSplitIterator

Embed an iterator in the CharSplitIterator struct, and combine that with
the former bool `only_ascii`; so use an enum instead.
Don't skip token after @'static

Fixes #8615.
Fix broken rusti

Commit 0932ab336 accidentally broke rusti by making it parse the binary
name as the input, instead of parsing what the user typed.
Android: Change directory before running tests
Android: Run benchmarks and copy metrics
Fix crash(!) by using the *Rust fn type* not the extern fn type

cc #3678
Change type of extern fns from `*u8` to `extern "ABI" fn`

cc #3678
Adjust callbacks in the libraries for the new type of extern fns

cc #3678
Refactor type combining to use default methods.  Woohoo!

This commit removes the "super_*" functions from
typeck::infer::combine, and adds them as default methods on the
Combine trait instead, making it possible to remove a lot of
boilerplate from the various impls of Combine.

I've been wanting to do this for over a year.  In fact, it was my
original motivation for default methods!

It might be possible to tighten things up even more, but this is the
bulk of it.