This commit adds functionality that allows the name resolution pass
to search for entities (traits/types/enums/structs) by name, in
order to show recommendations along with the errors.
For now, only E0405 and E0412 have suggestions attached, as per the
request in bug #21221, but it's likely other errors can also benefit
from the ability to generate suggestions.
for `~str`/`~[]`.
Note that `~self` still remains, since I forgot to add support for
`Box<self>` before the snapshot.
How to update your code:
* Instead of `~EXPR`, you should write `box EXPR`.
* Instead of `~TYPE`, you should write `Box<Type>`.
* Instead of `~PATTERN`, you should write `box PATTERN`.
[breaking-change]
