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/io.tri

@@ -96,6 +96,11 @@ onResult_ = action errCase okCase :
       (val _ : okCase val)
       result)
 
+mapErrIO prefix action =
+  onResult_ action
+    (e : pure (err (append prefix e) t))
+    (v : pure (ok v t))
+
 -- ---------------------------------------------------------------------------
 -- Convenience helpers
 -- ---------------------------------------------------------------------------