Nested function definitions don't have a frame in the `DEF_STORE`

[mark-koch]

I ran into this while working on monomorphisation. I worked around this for now, but the proper solution would be to add a frame to the store

They should probably just inherit the stack frame of the outer function

[acl-cqc]

Note that Hugr will be moving to "no nested FuncDefns" in a future release so it's probably not worth putting much effort in here

[mark-koch]

But I assume we still want them in Guppy?