rust / build: make install should use explicit permissions when creating directories

Closes #10046
extra: Add getter methods to extra::rational::Ratio

After merging 0ada7c7, user code have not been able to access to `Ratio`'s numerator and denominator fields.
In some algorithms, it is needed to get an rational number's numerator or denominator, but keeping these fields private is necessary for guaranteeing that `Ratio` numbers are irreducible.
So, I added the getter methods `numer()` and `denom()`.

As a bonus, this commit adds utility methods relating to the ratio-integer conversion.
libsyntax/librustc: Allow mut qualifier in patterns.
libsyntax: Get rid of some logic for some obsolete syntax.
Test for mut in ident patterns.
librustc: Don't emit unused mut lint twice.
libsyntax: Get rid of obsolete obsolete warning.
Fire fewer homing missiles

This optimizes the `home_for_io` code path by requiring fewer scheduler
operations in some situtations.

When moving to your home scheduler, this no longer forces a context switch if
you're already on the home scheduler. Instead, the homing code now simply pins
you to your current scheduler (making it so you can't be stolen away). If you're
not on your home scheduler, then we context switch away, sending you to your
home scheduler.

When the I/O operation is done, then we also no longer forcibly trigger a
context switch. Instead, the action is cased on whether the task is homed or
not. If a task does not have a home, then the task is re-flagged as not having a
home and no context switch is performed. If a task is homed to the current
scheduler, then we don't do anything, and if the task is homed to a foreign
scheduler, then it's sent along its merry way.

I verified that there are about a third as many `write` syscalls done in print
operations now. Libuv uses write to implement async handles, and the homing
before and after each I/O operation was triggering a write on these async
handles. Additionally, using the terrible benchmark of printing 10k times in a
loop, this drives the runtime from 0.6s down to 0.3s (yay!).
Remove ancient emacs mode lines from test cases

These are relics that serve no purpose.
lock-free queue for scheduler message queue
add cache line padding
add multi-producer multi-consumer bounded queue to use for sleeper list
add padding to prevent false sharing
clean up
fix bug introduced by previous clean up. more clean up.
minor
minor
Tidy
Add links to original mpmc and mpsc implementations
Encapsulate the lock-free mpsc queue in the MessageQueue type