Unsafe mod and foreign mod

[adetaylor]

No description provided.

[taiki-e]

(Note that parsing for these syntaxes is already supported (#928, #929). Adding a field is a breaking change, so currently parsed as Item::Verbatim.)

[adetaylor]

@taiki-e Thanks! Unfortunately I am generating code for input into https://github.com/dtolnay/cxx, so I was looking for full syn functionality. But you're right: I think I should generate Item::Verbatim too (I'm doing that in one other place, it's not too bad! - and I can probably do it by trivially modifying an ItemMod made by syn anyway)

It's clear from #928, #929 plus the PR builder results that this PR requires a bit more work. I'll close it for now and reopen it when it's spruced up, ready to land eventually next time there's a breaking release of syn.