isPure + Match Arm Elimination

[EmilySillars]

• adds a an isPure predicate to the preinline phase to account for side effects when inlining
• fixes a bug related to unintentional inlining of scrutinees, + added intentional inlining of scrutinees
• adds match arm elimination
• adds regression tests for each of the above 3 features

[EmilySillars]

This PR is ready to merge!

• adds a checkPure condition to the preinline phase to account for side effects when inlining
• fixes a bug related to unintentional inlining of scrutinees, + added intentional inlining of scrutinees
• adds match arm elimination
• adds regression tests for each of the above 3 features
• since the simplifier does not currently support mutual recursion, I added a check for recursive let bindings. Upon finding a let expression with a list of more than one binding, the simplifier passes along the program unmodified. I did this because I thought ignoring programs that potentially have mutually recursive bindings seemed like the easiest/simplest patch. @j-hui If you'd prefer I rename the mutual recursion regression tests to .failing instead, or some other solution, LMK.