Eliminate Reusing Mutable Terms

[FlandiaYingman]

The current compiler assumes that all Term are immutable and is reusing them. This assumption is wrong because of the resolution stage. This PR tries to fix the places where terms are reused.

See also #297 (comment).

AFAIK there are still two places where terms are reused and I couldn't fix them.

• During UCS desurgaring, the desugarer reuses the fallback split (e.g., in hkmc2/shared/src/test/mlscript/ucs/syntax/PlainConditionals.mls).

• Both Desugarer and Normalization share the same super class DesugaringBase. However, desugaring happens before resolution and normalization happens after resolution, so the helper functions that Desugarer uses should not initialize the implicit arguments when it creates new terms; while the helper functions that Normalization uses should initialize the implicit arguments. Unfortunately they use a same set of helper functions :-( Errors under hkmc2/shared/src/test/mlscript/rp are because of this).

[LPTK]

Thanks for your WIP! The pattern stuff is completely refactored in #316, so it's not worth dealing with here. Maybe we can just disable the check that makes these tests fail for now, and re-enable it later?

[FlandiaYingman]

The checks were not there before this PR, so I think we could just wait for Luyu's PR to be merged, and see how we want to change this PR according to that.