REPL namespaces; lib function for pattern matching

Adds support for REPL namespacing, primarily to avoid `main` collisions.
Also adds a library function for an ergonomic pattern matching function
that I've been noodling on. I might explore ways to make list syntax
less annoying specifically for pattern matching like this.

lib/base.tri

@@ -31,7 +31,7 @@ lOr = (triage
   (\_ _   : true)
   (\_ _ _ : true))
 
-matchPair = \a   : triage _ _ a
+matchPair = \a : triage _ _ a
 
 not? = matchBool false true
 and? = matchBool id (\_ : false)