Better error messages for the nullary-enum-variant-versus-variable binding problem.

[bblum]

The problem is, of course, as follows:

import names::{Alice, Bob};
...
match name {
    Alice => ...,
    Bob   => ...,
    Carol => ..., // <- treated as a variable because of forgotten import
    _     => ..., //~ WARNING unreachable pattern
}

or:

match name {
    Alice => ...,
    Bob | Carol => ..., //~ ERROR variable Carol not bound in all patterns
}

We should be able to emit a hint that says something like Maybe you forgot to import Carol?.

[jdm]

See #4612 and #4639.

[bblum]

woops, didn't do my homework before filing a duplicate.