Previously, warnings upon encountering a deprecated use `=` in
assignments, exports, and aliases would print a message without any
indication of where the offending `=` was. This diff adds a proper
`Warning` enum, and uses it to report context, as is done with
compilation and runtime errors.
Eventually, there will probably be a `crate` visibility specifier that
does the same thing as `pub(crate)`. This commit replaces `pub` with
`pub(crate)`, so when `crate` is available we can easily switch to it.
