Beside the new order being consistent with the ThreadPool API and making
more sense, this shuffling allows to write the context argument type in
terms of the startFn arguments, reducing the use of anytype (eg. less
explicit casts when using comptime_int parameters, yay).
Sorry for the breakage.
Closes#8082
* thread/condition: fix PthreadCondition compilation
* thread/condition: add wait, signal and broadcast
This is like std.Thread.Mutex which forwards calls to `impl`; avoids
having to call `cond.impl` every time.
* thread/condition: initialize the implementation
* move concurrency primitives that always operate on kernel threads to
the std.Thread namespace
* remove std.SpinLock. Nobody should use this in a non-freestanding
environment; the other primitives are always preferable. In
freestanding, it will be necessary to put custom spin logic in there,
so there are no use cases for a std lib version.
* move some std lib files to the top level fields convention
* add std.Thread.spinLoopHint
* add std.Thread.Condition
* add std.Thread.Semaphore
* new implementation of std.Thread.Mutex for Windows and non-pthreads Linux
* add std.Thread.RwLock
Implementations provided by @kprotty
