Revert "use StringRef where possible"

This reverts commit 370b29e29384b87bc2f5b8544a69a9545ed74dc3.

That commit broke 5 tests:

```
lang/modules/imports/ambiguous1: unexpected compiler error
lang/modules/imports/ambiguous2: unexpected compiler error
lang/modules/imports/ambiguous3: unexpected compiler error
lang/modules/imports/ambiguous4: unexpected compiler error
lang/modules/imports/ambiguous5: unexpected compiler error
```
fix Maybe(#1), attempt 2

fixes #493