Performance improvements w/Span API

Highlights

@AlgorithmsAreCool Added benchmarks for better visibility on performance #119
@AlgorithmsAreCool Introduced Span for much better native interop performance #122
Uses Github Actions and covers more OS variations
Updated native lmdb libs to 0.9.24

Upgrade instructions for those coming from pre-span changes.

MDBValue now makes itself more prominent in the public API. It exposes two new methods CopyToNewArray, and AsSpan. If you simply want older code to work and compile again, CopyToNewArray is what you're looking for. If you want to squeeze more performance, spend some time with AsSpan.