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

tests/TestMap.newt

@@ -1,5 +1,6 @@
 module TestMap
 
+import Prelude
 import SortedMap
 
 main : IO Unit