Possibility of no-std support

[indietyp]

I am contemplating removing the home-grown UnionFind structure of petgraph with ena.

We'd like to be able to provide petgraph in no-std; therefore: is there a possibility of ena being available as a no-std (but alloc) library?

[nikomatsakis]

Seems possible! We're not doing a lot of active maintenance of ena, but I'm open to landing PRs.