'let' within 'parallel' now works

tests/parallel/tlet_spawn.nim

@@ -0,0 +1,14 @@
+
+import threadpool
+
+proc foo(): int = 999
+
+# test that the disjoint checker deals with 'a = spawn f(); g = spawn f()':
+
+proc main =
+  parallel:
+    let f = spawn foo()
+    let b = spawn foo()
+  echo "done", f, " ", b
+
+main()