Add support for NSAutoreleasePool.

[maleadt]

This PR adds support for NSAutoreleasePool, and for running Julia code with an autorelease pool active. This is tricky for two reasons:

• autorelease pools are thread bound, while Julia tasks can migrate threads
• there's no way to switch autorelease pools, while Julia threads can switch between tasks

To work around both, I have a global lock to only ever have one piece of code running under an autorelease pool, while temporarily setting the current task's sticky bit to false to prevent thread migration.

[codecov]

Codecov Report

Attention: Patch coverage is 86.20690% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 72.06%. Comparing base (bbc8095) to head (b7b8c41).

Files	Patch %	Lines
src/foundation.jl	86.20%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #29      +/-   ##
==========================================
+ Coverage   69.84%   72.06%   +2.21%     
==========================================
  Files           9        9              
  Lines         630      655      +25     
==========================================
+ Hits          440      472      +32     
+ Misses        190      183       -7     

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