Proposal: improve the Data.Tree API

[Yitzchak Gale]

David Feuer wrote:
> A relevant change has occurred since this proposal came out: `length`
> got added to `Foldable`, with semantics that seem to match this
> `size`. In light of this, I think `size` should probably be dropped,
> and the `Foldable` instance expanded to give a better `length`.

Sounds good.

> Aside from that, someone just has to put together a pull request for
> haskell/containers on GitHub.

OK. João was the original owner; let's see if he wants to
follow through.

> The hardest part of this whole thing
> would be the module split. I don't personally see the point—trees are
> made of forests, which are made of trees, so while you *could* use
> Henning's trick to avoid cycles, you'd likely end up putting much of
> the code in Data.Tree.Private (to avoid orphan instances) and then end
> up with everyone exporting both public modules. For now, even with the
> proposed additions, Data.Tree is quite a small module, so I don't know
> why we should go to the trouble.

As discussed at the time - the consistent naming convention
throughout the containers library is to avoid using names of
types as parts of function names. Instead, separate modules are
used to qualify the same name for functions that do the same
thing. The module split was proposed to comply with that
convention, and no one expressed any opposition.

However, personally I'm also fine with using João's original
names to avoid the module split. What's important to me is
getting these long-overdue combinators into the library as
soon as possible, without arguing over the names.

Thanks,
Yitz