We already have a LICENSE file that covers the Zig Standard Library. We
no longer need to remind everyone that the license is MIT in every single
file.
Previously this was introduced to clarify the situation for a fork of
Zig that made Zig's LICENSE file harder to find, and replaced it with
their own license that required annual payments to their company.
However that fork now appears to be dead. So there is no need to
reinforce the copyright notice in every single file.
* 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
