implemented 'guard' annotation

tests/parallel/tguard1.nim

@@ -0,0 +1,37 @@
+
+when false:
+  template lock(a, b: ptr TLock; body: stmt) =
+    if cast[ByteAddress](a) < cast[ByteAddress](b):
+      pthread_mutex_lock(a)
+      pthread_mutex_lock(b)
+    else:
+      pthread_mutex_lock(b)
+      pthread_mutex_lock(a)
+    {.locks: [a, b].}:
+      try:
+        body
+      finally:
+        pthread_mutex_unlock(a)
+        pthread_mutex_unlock(b)
+
+type
+  ProtectedCounter[T] = object
+    i {.guard: L.}: T
+    L: int
+
+var
+  c: ProtectedCounter[int]
+
+c.i = 89
+
+template atomicRead(L, x): expr =
+  {.locks: [L].}:
+    x
+
+proc main =
+  {.locks: [c.L].}:
+    inc c.i
+    discard
+  echo(atomicRead(c.L, c.i))
+
+main()