Previously emoji like  (:man-man-boy:) were decoded as
:man::man::boy: () even though there is a specific emoji entry for
:man-man-boy:. The solution is to change how the regexp is constructed.
Instead of not setting a sort order for the emoji that go into the
regexp we use reverse alphabetical order to ensure that any emoji that
are inclusive of another (such as :man-man-boy: including :man) get
sorted BEFORE the emoji that they include.

We accomplish this by using a `SortedSet` and implementing `<=>` on `Emoji`

This is a fix for #7.
In addition to the updated iterator, I've adjusted the numeric emoji to
use accurate unicode codepoints.

A continuation of #3

In addition, this code rewrites encode to just defer to encode_io. This
is an attempt to fix #2, though I believe the lookahead code can and
should be refined.