Remove core-isa-parser package, prepare to transfer packages

[jessebraham]

• Eliminate the core-isa-parser package, and in turn the need for the xtensa-overlays submodule
• Fix some warnings in xtensa-lx-rt
• Add notice to README.md stating that the package has moved
  • I will open a PR adding xtensa-lx and xtensa-lx-rt to the esp-hal repo shortly

Tested on hardware on an ESP32-S3, still working as expected as far as I can tell.

Closes #40

[jessebraham]

I've opened a PR in esp-hal and transferred the two existing issues from this repo over to esp-hal, so once this is merged we can archive this repo.

[MabezDev]

Thanks!