Implement RFC 0002: ExecutionContext [EPIC]

[ysbaddaden]

Tracking issue to implement RFC #0002 that will introduce execution contexts into Crystal.

Draft

• RFC 2: ExecutionContext #15302 (all schedulers implemented)

Related pull requests to prepare for the actual implementation:

• Add Thread#internal_name= #15298
• Explicit exit from main #15299
• Add Thread::LinkedList#each to safely iterate lists #15300
• Fix: Make Crystal::EventLoop#remove(io) a class method #15282
• Fix: GC safety in Thread#start #14558 —appears to be wrong (segfaults)
• Add Crystal.print_buffered(io) and Crystal.print_error_buffered #15343
• Initialize Fiber with an explicit stack #15409

Actual pull requests that implement some of the RFC (to be extracted from #15302):

• Add fiber queues for execution context schedulers #15345
• RFC 2: Skeleton for ExecutionContext  #15350
• RFC 2 : Delay Fiber stack association #15404
• RFC 2: Add Fiber::ExecutionContext::SingleThreaded scheduler #15511
• RFC 2: Add Fiber::ExecutionContext::Isolated #15513
• RFC 2: MT safe fiber context switch on AArch64 #15581
• RFC 2: MT safe fiber context switch on ARM #15582
• RFC 2: Add Fiber::ExecutionContext::MultiThreaded #15517
• RFC 2: Add Fiber::ExecutionContext::Monitor #15599

Related issues (e.g. impacted by MT, or MT paradigm shift):

• ThreadLocalValue is not fiber-aware #15088
• Ensuring class_getter runs exactly once under concurrent access #14905
• Regression in untyped getter macro #15556

Fixes and thread safety

• Fix Crystal::EventLoop::LibEvent and FiberExecutionContext integration #15743
• Fix calling Fiber::ExecutionContext#enqueue from bare Thread #15767
• Fix: race condition in Fiber::ExecutionContext::Isolated#wait #15872
• Add Crystal::FdLock + integration #16128

Refinements and follow ups

• Add thread safety to at_exit #15598
• Update docs for Fiber::ExecutionContext.default_workers_count #15664
• Add Fiber::ExecutionContext::ThreadPool #15885
• PoC: detach execution context scheduler from running thread during blocking syscall #15871
• Rename execution contexts + improve their docs #15936
• Remove minimum in Fiber::ExecutionContext::Parallel #15946
• Add Fiber::ExecutionContext::Parallel#resize #15956
• Speed up Parallel::Scheduler#quick_dequeue? for max=1 #15961
• Lazily instantiate the event loop of isolated execution contexts #16063
• Drop custom implementation of Fiber::ExecutionContext::Concurrent #16135
• Default execution context is now parallel (MT:1) #16136
• Improve Fiber::ExecutionContext.default_workers_count #16149