Added home_for_io_with_sched variant. Temporarily making IO unkillable.
Allow metadata to be not compressed
Don't copy metadata after loading
Make enum discriminants u64 instead of the host uint.
Don't truncate discriminants to host uint for C_uint's sake.
Add a test case for the preceding changes.

This would have failed on 32-to-64-bit cross-compiles.
Don't ever compress metadata
std::str: Double-ended CharSplitIterator

Add new methods `.rsplit_iter()` and `.rsplitn_iter()` for &str.

Separate out CharSplitIterator and CharSplitNIterator,
CharSplitIterator (`split_iter` and `rsplit_iter`) is made double-ended
while `splitn_iter` and `rsplitn_iter` (limited to N splits) are not,
since these don't have the same symmetry.

With CharSplitIterator being double ended, derived iterators like
`line_iter` and `word_iter` are too.
bigint: cfg(target_arch = ...) => cfg(target_word_size = ...)
bigint: remove unnecessary method implements
bigint: inlining small functions
bigint: un-ignore test_shr
bigint: Add benchmarks
auto merge of #8723 : anasazi/rust/temporary-unkillable-io, r=brson

Also added a home_for_io_with_sched variant to consolidate some cases.

This is a temporary step to resolving #8674.
auto merge of #8745 : brson/rust/metadata, r=cmr

This does two things: 1) stops compressing metadata, 2) stops copying the metadata section, instead holding a reference to the buffer returned by the LLVM section iterator.

Not compressing metadata requires something like 7x the storage space, but makes running tests about 9% faster. This has been a time improvement on all platforms I've tested, including windows. I considered leaving compression as an option but it doesn't seem to be worth the complexity since we don't currently have any use cases where we need to save that space.

In order to avoid copying the metadata section I had to hack up extra::ebml a bit to support unsafe buffers. We should probably move it into librustc so that it can evolve to support the compiler without worrying about having a crummy interface.

r? @graydon
Add _opt variants to str byte-conversion functions

Add _opt variants to from_bytes, from_bytes_owned, and from_bytes_slice.
These variants return an Option instead of raising a condition/failing.
Revert "auto merge of #8745 : brson/rust/metadata, r=cmr"

This reverts commit 491bc3568c87dadaba4d342135bd308961c6e0ef, reversing
changes made to 05f1bbba16912f63b562a7847801823872f89ec6.
auto merge of #8751 : brson/rust/metadata-revert, r=brson

This reverts commit 491bc3568c87dadaba4d342135bd308961c6e0ef, reversing
changes made to 05f1bbba16912f63b562a7847801823872f89ec6.

I've heard reports that this broke something. Don't know details yet.
auto merge of #8750 : kballard/rust/str-opt-variant, r=thestinger

Add _opt variants to from_bytes, from_bytes_owned, and from_bytes_slice.
These variants return an Option instead of raising a condition/failing.
std::str: bench tests for .split_iter()