Support const generics in struct members and better diagnostics

Sanjayrequested to merge
ParallaxError/spade:const-generics-in-struct-members into main

Refactored constraint evaluation to be usable in mir_type_lowering, allowing const generics in struct members.

Refactored signatures related to constant evaluation to propagate errors and replaced assertions and panics with Diagnostic::bug

Caught division and modulo by zero during constant evaluation and raised a Diagnostic:error instead of BigInt panicking

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