Emojize all the things!
Optimization: Enumerate matches

The previous approach was to enumerate all emojis - the new approach checks the presence of the emojis in the text first and then only enumerates the matches
Optimization: Static emoji aliases

Previously the emoji dictionary got instantiated every time a string was transformed. Now the dictionary gets instantiated lazily the first it is needed and will be reused the next times.

As a bonus, this make the emoji aliases available to the outside via `[[NSString class] emojiAliases]` so this can be used i.e. for autocompletion.