fix: keep comptime let statements during elaboration

[guipublic]

Description

Problem*

Resolves Zac issue on using comptime let statements

Summary*

A let statement evaluates to a unit value, so replacing it with its evaluation is incorrect. Instead, we keep the evaluated statement, which should properly replace the rhs with comptime values.

Additional Context

Documentation*

Check one:

• [X] No documentation needed.
• [ ] Documentation included in this PR.
• [ ] [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• [X] I have tested the changes locally.
• [X] I have formatted the changes with Prettier and/or cargo fmt on default settings.

[guipublic]

I believe the issue is fixed by additional work on comptimes. Do you confirm that I can close this PR @jfecher?

[jfecher]

Confirmed this is fixed on current master in the elaborator at least