Trivial optimizations

[maemre]

There is some low-hanging fruit in terms of optimizations that we can (and should) implement:

• Tail calls, I think this is handled best by adding a tail call instruction in the IR.
• Constant folding and propagation.
• Eliminating check points when only one choice is possible (e.g. when pattern matching on a variable in different clauses that are mutually exclusive).
• Inlining small predicates.