Utilities for determining whether characters belong to character classes defined
by the XML specs.

## Organization

It used to be that the library was contained in a single file and you could just
import/require/what-have-you the `xmlchars` module. However, that setup did not
work well for people who cared about code optimization. Importing `xmlchars`
meant importing *all* of the library and because of the way the code was
generated there was no way to shake the resulting code tree.

Different modules cove