Prevent rustfmt from messing with verusfmt

[jaybosamiya]

Rustfmt messes with verusfmt's formatting, and this has been the cause of idempotency issues such as #33 and #36. With this PR, we close that out (hopefully) entirely.

In short, we add a new (minimal) parser that collapses any verus!{ ... } into verus!{} before passing it to rustfmt; upon getting a result from rustfmt, we expand out those verus!{} to verus!{ ... } again, thereby preventing rustfmt from messing with anything inside the body that we have carefully set up already.

This also allows us to enable the NR snapshot test, which is now stable. Fwiw, we could have fixed NR's non-idempotency if we could pass multiple macros to rustfmt's skip_macro_invocations config; however, that is blocked on an issue in rustfmt: rust-lang/rustfmt#6097; even once that is fixed, we would need to assume/check that the user is on recent-enough rustfmt (also it turns out that skip_macro_invocations is not stable on rustfmt), so the fix that this PR does is a much more manageable fix for us overall.

Additionally, this PR also adds in vstd as a snapshot test (which could not be added before, due to same idempotency issue).

Fixes #36

[parno]

Thanks for adding this. It's annoying to have to work around rustfmt this way, but it at least seems like it should definitively solve the problem.