UTF-8/Unicode Library

[davidzchen]

Given the importance of i18n in software and the lack any good Unicode support in C++, would you consider adding a Unicode library to Abseil?

Copies of the Google utf8 library has already shown up in various open source projects, such as the TensorFlow Syntaxnet model, and a number of other repos, and I think it would be great to make that a standard part of Abseil.