Fix a bug in `dbg` expression desugaring by using the module scope to
generate unique identifiers instead of the variable store.
In the initial implementation of `dbg` expressions we used the
`VarStore` to generate unique identifiers for new variables created
during desugaring. We should have instead used the current module's
`Scope`, which handles identifiers within the module. Each scope has its
own incrementing variable count which is independent of the shared
variable store. The scope is used to generate new identifiers at other
points in canonicalization, such as when assigning a global identifier
to closures and `expect`s. It's possible that the identifier generated
for `dbg` could conflict with an identifier generated by the scope,
resulting in a confusing error.
