UTF-8/Unicode Library

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](https://github.com/tensorflow/models/tree/1f61912ac400094290b32510c3b9e26f2b36ebbf/research/syntaxnet/util/utf8) model, and [a number of other repos](https://github.com/search?l=C%2B%2B&p=1&q=org%3Agoogle+unicodetext&type=Code), and I think it would be great to make that a standard part of Abseil.