This patch introduces a `CombiningClass` type to represent the canonical
combining class of a codepoint. The `fmt::Display` implementation of
this type writes the human-readable name of the combining class, if
there is one. This replaces the previous behaviour, which was to just
print the raw byte value of the combining class.
This patch adds `clap` as a dependency for command-line argument
parsing, and introduces a `-f` flag to allow toggling whether or not the
full names of character classes are displayed.
pantonshire