0.4.0 alpha 2

Second pre-release on the path to 0.4. Mostly adds typechecking improvements, along with "flags" to make it easier to inspect BPK types.

What's Changed

• Add flags to record endianiess and mappability by @mdekstrand in #48
• Use Pyright for typechecking by @mdekstrand in #49

Full Changelog: v0.4.0a1...v0.4.0a2