Ergonomic language features and lib cleanup

+ let bindings
+ where bindings
+ do notation

I explored enough of the alternative language design space and decided
that we should commit fully to Lambda style. That means no more highly
tacit/concatenative point-free/partial programs as default. We'll keep
taking advantage of those capabilities when it makes sense, but the
library will continue to see massive overhauls.

lib/patterns.tri

@@ -1,5 +1,6 @@
 !import "base.tri" !Local
 !import "list.tri" !Local
+!import "lazy.tri" !Local
 
 match_ = y (self value patterns :
   triage
@@ -22,3 +23,20 @@ match = (value patterns :
     patterns))
 
 otherwise = const (t t)
+
+cond_ self patterns =
+  lazyList
+    (_ : t)
+    (pattern rest :
+      matchPair
+        (testK actionK :
+          lazyBool
+            actionK
+            (_ : self rest)
+            (testK t))
+        pattern)
+    patterns
+
+cond patterns = y cond_ patterns
+
+guard testK actionK = pair testK actionK