[m-rev.] for review: provide alternatives to can-fail functions in the library

[Zoltan Somogyi]

For review by anyone.

This diff also adds some "XXX STATEVAR"s to library/injection.m.
The marked predicates have non-statevar-friendly argument orders.
Given that injection.m is rarely used, I would like to simply change
those arguments orders in a follow-up diff, noting the change in NEWS.md.
Does anyone think that we need a more gradual implementation
of that change?

Something that would definitely have more impact is replacing the semidet
function in the enum typeclass. (The uenum typeclass already has a
semidet *predicate* in the same role.) Since you cannot obsolete one
typeclass method in favor of another, I can't think of a way to make any
changeover any less disruptive than just making the replacement and announcing it,
and expecting everyone to fix their instance declarations when they next update
their Mercury standard library. Can anyone else?

Zoltan.
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