Removing `leaves` export

[theogf]

It is currently quite inconvenient that BenchmarkTools.jl export the leaves function as it is a very common name and tend to clash with a lot of other packages containing Trees.

Would you consider a (breaking) PR which would remove this specific export?

[gdalle]

We're heading towards a 2.0 release so it's now or never. Can you maybe check the history of the repo to see whether they list good reasons for the export (issues, PRs, commits)?

[theogf]

I could not find any discussion on it (PR or issues). It has a very clear use (iteration of a BenchmarkGroup), I guess the name clash was just never really considered.

Another option would be to depend on AbstractTrees.jl but that's a very unnecessary dependency.

[lucaferranti]

I also was hit by this a few times and I think it would make sense not to export it.

It's good to mention also that atm the function does not have a docstring and is not included in the References section of the docs. Based on the short example in the manual it looks like something most end users would not need? (At least I dont recall needing it).

[gdalle]

Wanna make a PR? Should be a quick review

[lucaferranti]

Can open one tomorrow morning if nobody ninjas me by then :)