The `Digest::CRC12_3GPP` class is not available

[dearblue]

There is a C extension ext/digest/crc12_3gpp/crc12_3gpp_ext.c file, but the Ruby implementation (lib/digest/crc12_3gpp.rb file) is missing and consequently unavailable.
There is also no corresponding spec/crc12_3gpp_spec.rb file.

However, there is no description in README.md either, so there seems to be no problem even if it is not available.