[m-rev.] for post-commit review, and a proposal: sparse_bitsets

[Zoltan Somogyi]

The diff is for review by anyone.

The proposal is to add a new typeclass to enum.m.
At the moment, we have this one:

:- typeclass enum(T) where [
    func to_int(T) = int,
    func from_int(int) = T is semidet
].

The proposal is to add this one:

:- typeclass enum_uint(T) where [
    func to_uint(T) = uint,
    pred from_uint(uint::in, T::out) is semidet
].

The diff explains why the new typeclass would be better
for sparse bitsets, and probably for other applications as well.
In most and maybe all of our use cases, the to_int function
never returned any negative numbers anyway, so switching
uses of sparse_bitsets in the compiler to a version using
this typeclass would be trivial.

Would anyone object to

- adding the new typeclass,
- its switch from a semidet function to a semidet predicate, or
- switching {fat_}sparse_bitset.m to using the new typeclass?

The last would be an incompatible change, but we could mention in NEWS
that users could replace any to_int function with a to_uint function like this:

to_uint(Item) = UInt :-
  Int = to_int(Item),
  ( if Int < 0 then UInt = (-Int) + 1 else UInt = from_int(Int) * 2 ).

This represents negative ints as odd uints and non-negative ints
as even uints.

And does anyone have a better name than enum_uint?

Zoltan.
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