Make fields of mir::Terminator public
When trying to use the RA crate, I am unable to access the fields in `hir_def::mir::Terminator`.
I don't see any reason, why these should be private, especially as the fields of `hir_def::mir::Statement` are `pub`.
I am not sure if the fields in `hir_def::mir::SwitchTargets` should be made `pub` too, but at least they are read-accessible via some public methods..
Sorry if I missed something, this is my first PR.
f6ae0c11ac