Eliminate indexing errors in compiler

[joelberkeley]

We use List and Nat for stack and pointer, respectively. This means we have to handle indexing errors, which risk failing at runtime, and require either use of idris_crash/die, or (slow?) EitherT monad. If instead we use Vect n and Fin n, we can remove this problem at compile time. This would probably require changing the way we handle nested functions, but that might be a good thing on its own.

[joelberkeley]

This might be impossible as we don't want to expose this indexing in the user API