Add Unicode.replace_invalid/2

[Moosieus]

Thinking out loud here:

• Is it worth keeping the notebooks folder + single Livebook? I figure there's an argument for documenting the institutional knowledge, and also an argument for keeping extraneous stuff out of the repo.
• I considered adding benchmarks for future implementations to compare against. I figure that can be done if/when one presents itself.
• I didn't get around to identifying what gets replaced in the return value, but I think that's an endeavor unto itself.
• I'm content with the test cases for now - I even identified a few bugs in the implementation using them! If OTP ever receives a native implementation of this feature, perhaps they can use these tests as well.
• I didn't write tests for UTF16 and UTF32, but their implementations are pretty simple anyway.

Otherwise, I think this feature's ready to go!

[kipcole9]

Cameron, that's great work, thank you. Will review asap but I may be a bit challenged to do that today.

[kipcole9]

I've just published Unicode version 1.18.0 with the following changelog entry:

Enhancements

• Adds Unicode.validate_utf8/3 to force-validate a binary as a UTF-8 string. Any invalid codepoints or incomplete sequences are replaced with a replacement string. Many thanks to @Moosieus for the contribution.

Thanks very much again for the contribution, its greatly appreciated.