Recursive definitions

[brendanzab]

For Pikelet to be of much use at all, we should allow definitions to be recursive. I would like to add totality checking at some stage, like Idris has. Mutually recursive definitions would also be handy!

[brendanzab]

Perhaps as a bodge, in the interim we could keep track of the depth of the recursion. But it would be nice to have proper totality checking in the future. That would take a bit more work however (in terms of getting our heads around the theory).