Exploring a new definition of 0.5.0

With the incoming rust implementation, a pretty substantial boost to the Python implementation, and a big test suite that might require making a lot of small but subtle incompatibilities on the JS side, I'm considering boosting us to 0.5.0.

I mentioned before that I was aiming for a grammar spec in 0.5.0, but frankly, I suspect that an ABNF spec is not the most important thing to the project right now, given the Lark grammar and the test suite, so I'm happy to reject that goal for 0.5.0 in favor of what feels like a useful, substantial release.

Rough changelog I'm aiming for for 0.5.0:
1. A new rust implementation
2. Python lazy mode
3. Better cross-platform consistencies (aka restricting JS to validate more strongly)