Rework module system

Don't require/allow naming a module, instead require that the importer
names it. Allow importing into the local scope with the name !Local.
Simplify namespacing logic. Updates all tests to reflect these changes.

demos/size.tri

@@ -1,24 +1,22 @@
-!module Size
-
-!import "lib/base.tri" Lib
+!import "lib/base.tri" !Local
 
 main = size size
 
 compose = \f g x : f (g x)
 
-succ = Lib.y (\self :
-  Lib.triage
+succ = y (\self :
+  triage
     1
     t
-    (Lib.triage
+    (triage
       (t (t t))
-      (\_ Lib.tail : t t (self Lib.tail))
+      (\_ tail : t t (self tail))
       t))
 
 size = (\x :
-  (Lib.y (\self x :
+  (y (\self x :
     compose succ
-      (Lib.triage
+      (triage
         (\x : x)
         self
         (\x y : compose (self x) (self y))