bitformat-0.2.1

This release replaces the core bit manipulation code with a version written in Rust. This should allow some great optimisations in the future, but for now it's more a proof of concept to get the interface and the build systems working.

Some other changes are still a bit half-baked. There is code to switch from my hand-rolled format parsing to using the lark library. There are additions for new fields such as Repeat, If and Pass, and some work has been done on expressions. All of this is rather unfinished and planned for the 0.3 release, while this release concentrates on the move to use a Rust backend.

Pre-built wheels are available for some common platforms - these will be expanded later. Otherwise you'll need a Rust installation to build the library yourself on installation.

Other additions:

• Reader -- A new class that wraps a Bits to provide a bit position and read / parse functionality.