Issue #352Closes#1720
The old checker would happily accept things like 'alt x { @some(a) { a } }'.
It now properly descends into patterns, checks exhaustiveness of booleans,
and complains when number/string patterns aren't exhaustive.
Return the result of the discriminant from trans_alt,
rather than nil, in the _|_ case. This was breaking the
enclosed test case (alt-bot-2) when optimization was disabled.
Closes#769
