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/atomic/queue.zig

@@ -16,7 +16,7 @@ pub fn Queue(comptime T: type) type {
     return struct {
         head: ?*Node,
         tail: ?*Node,
-        mutex: std.Mutex,
+        mutex: std.Thread.Mutex,
 
         pub const Self = @This();
         pub const Node = std.TailQueue(T).Node;
@@ -27,7 +27,7 @@ pub fn Queue(comptime T: type) type {
             return Self{
                 .head = null,
                 .tail = null,
-                .mutex = std.Mutex{},
+                .mutex = std.Thread.Mutex{},
             };
         }