MassaLabs: Eliminate exponentiations in the IR

[Leo-Besancon]

This PR completes #333, and removes the Operation::Exp node in the IR. Instead, parsed exponentiations are reduced to a series of multiplications using square and multiply.

[bitwalker]

LGTM - the clippy errors need to be fixed, but I'll do that in my functions branch, and then we can rebase on top of that to get a green build again

[bitwalker]

@Leo-Besancon I've addressed the clippy warnings in next with the merge of #351, however we have some conflicts, so you'll have to rebase. I don't think there's actually any significant conflicts, just overlapping disjoint changes in the same files, but I haven't checked. Let me know if you have any questions about how to resolve any conflicts you aren't sure about.

[bobbinth]

Looks great! Thank you! My only one suggestion would be to modify one of the tests somewhere so that we see how expanding something other than squaring works (e.g., have x^7 somewhere).

[Leo-Besancon]

Thank you @bitwalker and @bobbinth, I resolved the small conflicts and updated the complex function test to test the 7 case.

[bobbinth]

Looks good! Thank you! I left one comment inline - but this is more for the time when we work on the codegen side. Could you create an issue based on it?