IR-level optimizations

[klntsky]

Today I was looking into purescript-backend-optimizer. It does a lot to optimize pattern matching, but we can't simply adapt the code: it uses own domain types. The main value proposition of corefn optimizer is reorganization of pattern matching in order to not make the same dispatches again. This is not much for us: this optimization, after all, can be done on the language level, sometimes for the price of less readable code. Let's skip this as unnecessary for a prototype.

There are optimizations involving record updates, we can forget about them, because our target is not JavaScript.

And another group of optimizations is related to fusion of arrays.

TBD: a summary