Equinox v0.3.0

Three main things in this release.

1. equinox.experimental.BatchNorm. Hurrah, that's nice to have.
2. Very interesting from a technical point of view: stateful operations. In this case, equinox.experimental.{get_state, set_state, StateIndex}. These are the technology used to update the statistics of BatchNorm without requiring the user to faff around outputting the model themselves. They work by wrapping jax.experimental.host_callback.call to save and load external state on demand. Which is pretty magic, so these should really be used sparingly...
3. Removed several old pieces of deprecated functionality: equinox.jitf and so on.

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Autogenerated release notes as follows:

What's Changed

• Score based example by @patrick-kidger in #42
• minor doc fixes by @patrick-kidger in #43
• Version 0.3.0 -- BatchNorm and stateful by @patrick-kidger in #50

Full Changelog: v0.2.2...v0.3.0