Add partial evaluator

[jfecher]

Problem

As noir grows we naturally want to add more and more comptime features to it, yet arbitrary features can be difficult to add to the SSA pass and it seems unnecessary to have SSA manage all these extra features which should always be removed at compile-time.

Solution

If we added a partial-evaluation pass to inline functions/loops before SSA and evaluate comptime expressions then it would make several features easier to add in the future: expanding comptime expressions, the mini-evaluator in the name resolver can be removed, and for-loops returning arrays can be implemented.

Alternatives considered

Alternatively, we can add these features all to SSA and deal with them there. Depending on the feature this may be more difficult to add.

[kevaundray]

Please add a refactoring label and remove refactoring from the title

[jfecher]

Closing this issue. We've experimented with a partial evaluator somewhat but have elected to keep comptime evaluation in SSA which can better evaluate across functions during inlining.