Add call arity analysis to CakeML

This issue is about implementing what Haskell calls arity analysis. https://www.joachim-breitner.de/publications/CallArity-ICFP2015-preprint.pdf.
I think this will involve a pass that just does eta expansion of functions before getting a more general inliner to inline the eta-expanded functions.
I suspect that the correctness proofs where Haskell ensure the condition of no extra work is being done for performance reasons will be a requirement in CakeML due to potentially effectful functions.

Start by reading the linked Call Arity paper and the CakeML compiler documentation or relevant compiler entry points. Determine how arity analysis, eta expansion, inlining, and the proposed correctness conditions fit CakeML's semantics. Done would require an implemented analysis and pass together with the required correctness proofs.

Written by the indexing model from the issue text.