Report duplicate alt patterns as errors

[jdm]

Being able to write something like

alt (value) {
_ { log "wheee!" }
_ { fail }
}

is silly. This branch doesn't have any tests included at the moment, however, because any non-trivial case butts up against the FIXME in trans_alt.rs that causes an error if any alt statement contains unreachable branches.

[marijnh]

This is fine (I don't have time to fully review and pull just now), but at some point it'll have to be smarter than just duplicate patterns, and also detect stuff like this:

alt foo {
   _ { /* this catches everything */ }
   somevariant(?x) { /* unreachable */ }
}

Also, I'll make a note to fix the trans_alt FIXME this week. It shoiuldn't even be hard, I just forgot about it when I pushed that change.

[brson]

jdm: I think marijnh has fixed that issue with translating unreachable branches. Can you add a test case?

[marijnh]

I just pushed beab6ba , which does this in a more robust way (as described in my previous comment on this pull req).