Rename mm to moduleinst

[rossberg]

The seemingly innocent alpha-renaming of a local variable in this PR causes a stack overflow in the interpreter:

File "test-interpreter/dune", line 1, characters 0-285:
 1 | (mdx
 2 |   (libraries spectec)
 3 |   (deps
....
12 |   )
13 |   (files TEST.md)
14 | )
Fatal error: exception Stack overflow

@f52985, @ShinWonho, could one of you please have a look?

[ShinWonho]

The problem was that we hardcoded the return instruction of module instantiation, and the variable "mm" is also hardcoded. I think this is one of the part that is hard to automate, so I simply renamed the variables.

[rossberg]

Thanks! So, if I understand the change correctly, this only concerns the variable name in $instantiate?

Do you think we could have some suitable checks and error messages for assumptions like this? Just silently running with misbehaving code when a user fails to match undocumented backend assumptions is not the best user experience. ;)

[ShinWonho]

Yes, it was the case in $instantiation, but now I find that we can remove the hardcoded variable name here. (Now you can freely use "mm", "moduleinst", or anything else) However, for checks and error messages, it is not a trivial things. I have several things in mind, but maybe I need to discuss it with Donjun and other people at KAIST.