an attempt at a singleton pausible idle callback for each scheduler. suffers from nondeterministic deadlock and also pending scheduler messages on scheduler shutdown.
std::rt: Touch up idle logic
std::rt: Fix a race in UvRemoteCallback's dtor that misses callbacks

Full description in comments.
Moved the logic for a pausible idle callback into a new type - PausibleIdleCallback and placed the appropriate signatures in rtio and implementation into uvio.
A round of code cleaning for the primary scheduler code. Comments have been updated, a minor amount of support type restructing has happened, methods have been reordered, and some duplicate code has been purged.
Rangechange the log message truncation limit.
Make supertrait methods callable on object types.

This requires changes to method search and to codegen. We now emit a
vtable for objects that includes methods from all supertraits.
Closes #4100.

Also, actually populate the cache for vtables, and also key it by type
so that it actually works.
clean whitespace :/
std: Restore dynamic borrow tracking
Upgrade llvm to current HEAD

* This has one workaround patch (everything's testing just fine...)
* I reworked the fixedstacksegment attribute to be specified with a string
  rather than using a keyword and an integer and modifying the parser
* I added a "no-split-stack" attribute along the same lines as the
  "fixedstacksegment" attribute for #1226
auto merge of #8463 : brson/rust/borrowck, r=nmatsakis,nikomatsakis

r? @nikomatsakis
Fix LLVM compilation issues and use the new attrs

This implements #[no_split_stack] and also changes #[fast_ffi] to using the new
"fixedstacksegment" string attribute instead of integer attribute.
auto merge of #8566 : toddaaro/rust/idle-opt+cleaning, r=catamorphism,brson

Instead of a furious storm of idle callbacks we just have one. This is a major performance gain - around 40% on my machine for the ping pong bench.

Also in this PR is a cleanup commit for the scheduler code. Was previously up as a separate PR, but bors load + imminent merge hell led me to roll them together. Was #8549.
auto merge of #8328 : alexcrichton/rust/llvm-head, r=brson

The first commit message is pretty good, but whomever reviews this should probably also at least glance at the changes I made in LLVM. I basically reorganized our pending patch queue to be a bit more organized and clearer in what needs to go where. After this, our queue would be:

* Add the `no-split-stack` attribute
* Add the `fixedstacksegment` attribute
* Add split-stacks for arm android
* Add split-stacks for arm linux
* Add split stacks for mips

Then there's a patch which I added to get rust to build at all on LLVM-head, and I'm not quite sure why it's there, but nothing seems to be crashing for now! (famous last words).

Otherwise, I just updated code to reflect the changes I made in LLVM with the only major change being the advent of the new `no_split_stack` attribute. This is work towards #1226, but someone more familiar with the code should probably actually assign the attribute to the appropriate functions.

Also as a bonus, I've verified that this closes #5774
auto merge of #8519 : msullivan/rust/objects, r=catamorphism

r?
std: bootstrapping libuv-based fileio in newrt... open & close

the test "touch"es a new file
std: working tests for low-level libuv open, write and close operations
std: add read and unlink to low-level FileDescriptor + end-to-end CRUD test
std: support async/threadpool & sync paths in uv_fs_* calls + add sync test
std: remove fcntl const bindings + making valgrind clean w/ no owned vecs