organize std lib concurrency primitives and add RwLock

* 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

src/ThreadPool.zig

@@ -6,14 +6,14 @@
 const std = @import("std");
 const ThreadPool = @This();
 
-lock: std.Mutex = .{},
+lock: std.Thread.Mutex = .{},
 is_running: bool = true,
 allocator: *std.mem.Allocator,
 workers: []Worker,
 run_queue: RunQueue = .{},
 idle_queue: IdleQueue = .{},
 
-const IdleQueue = std.SinglyLinkedList(std.ResetEvent);
+const IdleQueue = std.SinglyLinkedList(std.Thread.ResetEvent);
 const RunQueue = std.SinglyLinkedList(Runnable);
 const Runnable = struct {
     runFn: fn (*Runnable) void,