Modernize ~4 year old code

I modernized the implementation, moving to Rust 2021. Too many minor improvements to list here. Updated the version of most dependencies, except petgraph.

I deprecated IdTable which should be an implementation detail, and EntryIterable which is needlessly unsafe.

I plan to overhaul the API in the v0.3 implementation, including optional support for [brie trees], and implementing an IdManager (related functionality).

Barring an unforeseen issue, this should be the last release of the v0.2 series.