fix!: Disable automatically mapping a Dafny string to a Go string in externs

[robin-aws]

Fixes #2989. From my comments on that issue:

The root cause is that the Go compiler tries to special-case extern declarations that accept or return string values, attempting to directly map the Dafny string type to the native Go string type, and coerce between them. The problem is that the SpecialNativeType subclass of UserDefinedType used to represent the native Go string type doesn't have all the expected values for a resolved type, which caused the above assertion failure if assertions are checked. In the release build, the symptom is just failing to recognize that two types are actually the same.

For the record, the root cause here is that this feature is not deeply and rigorously implemented, and to my knowledge none of the other compilers attempt to support this style of automatic equivalence of types for externs. My strong preference is to remove the implementation, being as careful as necessary about the fact that it is a breaking change.

I attempted to rewrite the one test that demonstrates this functionality (GoModule.dfy) to include manual conversions between the two string types, but I ran into several unrelated issues, and so downgraded the test to a negative test for now under Test/wishlist.

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.