Add codepoint utility functions that test whether a codepoint belongs
to a set of codepoints (e.g., the valid codepoints for a URL scheme).
Most of these functions are implemented using a compact lookup table
so should be reasonably fast.
The functions are based on similar functions from the css-parser branch
that were introduced in commit 1698324920. We'll want to merge these
sets of functions together once the branches are merged.
