std.Thread: Mutex and Condition improvements

[kprotty]

Main Goals

• Optimize Mutex and Condiiton implementations to use Futex based algorithms.
• More robust tests for Mutex and Condition.
• More documentation in general, but specifically about Condition semantics.

Side Goals

• Guarantee at most u64 large sync primitives (if Futex is removed in the future, they can always be made usize large).
• Introduce std.Thread.Futex.Deadline for accurately waiting on a Futex in a loop with a relative timeout.
  • currently used by Condition's futex impl.
  • will be used by Semaphore and ResetEvent's futex impl.

[andrewrk]

error: <inline asm>:1:12: invalid operand for instruction
        lock btsl (%rax), %eax
                  ^~~~~

Is this a problem with stage2 inline assembly? If so, I can work on this to unblock you asap. However if it's possible to work around this with non-assembly code, you can check for @import("builtin").zig_backend != .stage1 and do the fallback condition. Then I can remove the fallback in favor of the assembly after the stage2 issue is fixed, at my leisure.

[kprotty]

@andrewrk Seems like stage2 either doesn't properly lower the *p/*m constraint or treats the X constraint incorrectly.

Also, the CI also says noalias pointers aren't actually pointers. Is this a stage2 false positive?

[gracefuu]

LGTM! I can't really comment on the platform specific implementations, but the FutexImpls seem fine. Also, the Condition tests are quite good and can be reused for Semaphore when the time comes to update those.

Just some really minor typos/comments: