Update casing to match compiler

[AlexBMJ]

Summary

It looks like casing.py in lib was not updated to match the newer version from betterproto-compiler, when the compiler was merged back into this repo. This patch just copies the compiler version into lib.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
  • This change has an associated test.
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[AlexBMJ]

This PR fixes #156