extract ThreadPool and WaitGroup from compiler to std lib

lib/std/Thread/WaitGroup.zig

@@ -0,0 +1,46 @@
+const std = @import("std");
+const Atomic = std.atomic.Atomic;
+const assert = std.debug.assert;
+const WaitGroup = @This();
+
+const is_waiting: usize = 1 << 0;
+const one_pending: usize = 1 << 1;
+
+state: Atomic(usize) = Atomic(usize).init(0),
+event: std.Thread.ResetEvent = .{},
+
+pub fn start(self: *WaitGroup) void {
+    const state = self.state.fetchAdd(one_pending, .Monotonic);
+    assert((state / one_pending) < (std.math.maxInt(usize) / one_pending));
+}
+
+pub fn finish(self: *WaitGroup) void {
+    const state = self.state.fetchSub(one_pending, .Release);
+    assert((state / one_pending) > 0);
+
+    if (state == (one_pending | is_waiting)) {
+        self.state.fence(.Acquire);
+        self.event.set();
+    }
+}
+
+pub fn wait(self: *WaitGroup) void {
+    var state = self.state.fetchAdd(is_waiting, .Acquire);
+    assert(state & is_waiting == 0);
+
+    if ((state / one_pending) > 0) {
+        self.event.wait();
+    }
+}
+
+pub fn reset(self: *WaitGroup) void {
+    self.state.store(0, .Monotonic);
+    self.event.reset();
+}
+
+pub fn isDone(wg: *WaitGroup) bool {
+    const state = wg.state.load(.Acquire);
+    assert(state & is_waiting == 0);
+
+    return (state / one_pending) == 0;
+}