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.

src/Main.hs

@@ -74,7 +74,9 @@ readEvaluatedForm = eitherReader $ \s -> case s of
   "ternary" -> Right Ternary
   "ascii"   -> Right Ascii
   "decode"  -> Right Decode
-  _         -> Left $ "Unknown format: " ++ s ++ ". Expected: tree, fsl, ast, ternary, ascii, decode"
+  "number"  -> Right Number
+  "string"  -> Right StringLit
+  _         -> Left $ "Unknown format: " ++ s ++ ". Expected: tree, fsl, ast, ternary, ascii, decode, number, string"
 
 evalParser :: Parser TricuArgs
 evalParser = Eval
@@ -84,7 +86,7 @@ evalParser = Eval
      <> short 'f'
      <> metavar "FORM"
      <> value Tree
-     <> help "Output format: tree, fsl, ast, ternary, ascii, decode"
+     <> help "Output format: tree, fsl, ast, ternary, ascii, decode, number, string"
       )
   <*> option str
       ( long "output"