Without a pre-built lookup, we will look through all existing emojis (~1000) for non-existent emoji cheat codes. This is 30x faster than caching on demand in the presence of non-existent cheat codes, and roughly the same speed as caching on demand when there are no non-existent cheat codes.

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

Fixes #1

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.

I love these descriptions