Add `FieldInfo::field_type_name`

[workingjubilee]

In my case, I want to make a field private and wrap it behind an accessor function that makes the field unsafe to access. I need the field's type in order to make this decision correctly. In some cases a type name is not available and I am fine with that.

[pvdrz]

Is there any chance you could document this in the changelog? it looks good to me otherwise

[bors-servo]

:umbrella: The latest upstream changes (presumably 9a8e5ca2f4089e21300fbea6b73e82efd7f18882) made this pull request unmergeable. Please resolve the merge conflicts.