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/patterns.tri

@@ -0,0 +1,35 @@
+!import "list.tri" !Local
+
+match_ = y (\self value patterns :
+  triage
+    t
+    (\_ : t)
+    (\pattern rest :
+      triage
+        t
+        (\_ : t)
+        (\test result :
+          if (test value)
+             (result value)
+             (self value rest))
+        pattern)
+    patterns)
+
+match = (\value patterns :
+  match_ value (map (\sublist :
+    pair (head sublist) (head (tail sublist)))
+    patterns))
+
+otherwise = const (t t)
+
+-- matchExample = (\x : match x [[(equal? 1)  (\_ : "one")]
+--                               [(equal? 2)  (\_ : "two")]
+--                               [(equal? 3)  (\_ : "three")]
+--                               [(equal? 4)  (\_ : "four")]
+--                               [(equal? 5)  (\_ : "five")]
+--                               [(equal? 6)  (\_ : "six")]
+--                               [(equal? 7)  (\_ : "seven")]
+--                               [(equal? 8)  (\_ : "eight")]
+--                               [(equal? 9)  (\_ : "nine")]
+--                               [(equal? 10) (\_ : "ten")]
+--                               [ otherwise  (\_ : "I ran out of fingers!")]])