`MacroCallLoc` and `MacroDefId` should live in the `span` crate

These are effectively our version of rustc's `HygieneData` and we'll need them in the MBE crate to resolve hygiene. The problem with that is that we currently store `Subtree`s in `MacroCallLoc` which can't and should not be part of the `span` crate for obvious reasons (and they should not be in `MacroCallLoc` in the first place).