fix: reduce let-bodies correctly in `StructInst` (#3152)

Closes #3146

Reduction doesn't trigger correctly on the bodies of `let`-expressions
in `StructInst`, leading some meta-variables to linger in the terms of
some fields. Because of this, default fields may try multiple times (and
fail) to be generated, leading to an unexpected error.

The solution implemented here is to modify the values of the introduced
variables in the local context so as to reduce them correctly.