performance and code size improvements

- Use default case for constructors with no explicit match.
- self-compile is 15s now
- code size is 60% smaller

code size and self compile time on par with the idris-built version

newt/Prelude.newt

@@ -343,7 +343,7 @@ IO a = World -> IORes a
 instance Monad IO where
   bind ma mab = \ w => case ma w of
     MkIORes a w => mab a w
-  pure x = \ w => MkIORes x w
+  pure = MkIORes
 
 bindList : ∀ a b. List a → (a → List b) → List b