Remove a fixme

I don't think it's relevant, a warning is more appropriate. Easier to just
maintain the order.

Closes #4681
Add tidy support for snapshots

This let's us specify exactly which snapshot a given note to update after
snapshot is for.

Closes #2483
Remove a fixme

pcwalton says this is right, and it looks right to me too.

Closes #4731
tidy: spew less chaff
std: mark some modules as unstable

Obviously everything is unstable, but these particularly so, and they will
likely remain that way.

Closes #10239
Fix some warnings
auto merge of #11333 : cmr/rust/triage2, r=alexcrichton