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

lib/std/Progress.zig

@@ -50,7 +50,7 @@ done: bool = true,
 /// Protects the `refresh` function, as well as `node.recently_updated_child`.
 /// Without this, callsites would call `Node.end` and then free `Node` memory
 /// while it was still being accessed by the `refresh` function.
-update_lock: std.Mutex = .{},
+update_lock: std.Thread.Mutex = .{},
 
 /// Keeps track of how many columns in the terminal have been output, so that
 /// we can move the cursor back later.