Bidirectional UUID mapping for cracked accounts

# Contextualization
Following issue #45, it is now more important than ever to raise awareness to future users of the API to save player-related data using unique ids, instead of player names.

Names are not only variable since premium accounts can change them, they're also no longer unique as well, since players with the same name (but different account type) can join the server simultaneously.

# Problem
Linking data to the unique id is the safest option, and it can be easily queried by converting the unique id to the corresponding name.

For original accounts, this is easily done by consulting Mojang's API.

For pirated accounts, however, that feat is impossible. Unique ids for pirated accounts are generated from the hashed MD5 string `OfflinePlayer:`, and so, are unidirectional functions, easily allowing the calculation of a unique id based on a name, but making the opposite impossible.

# Suggestion
A new unique id mapping is suggested, allowing for bidirectional calculation. Below are two suggested implementations, each one having its advantages and disadvantages.

## Implementation 1
Unique ids are composed of 128 bits, with 6 of them being reserved (4 for the version and 2 for the variant), resulting in 122 usable bits of data. By padding the user's name with `null` characters to the left, we can encode each of the 16 letters using the same amount of bits, or 7 (7.625 rounded down) per letter in this case. With 7 bits per letter, it is possible to represent every possible character in the ASCII table, accounting as well for every valid character (`[0-9A-Za-z_]`).

- **Advantage:** Flexibility in case new valid characters are added in the future.
- **Disadvantage:** No flexibility in case the maximum length of names is increased.

## Implementation 2
There's a total of 63 possible options for each letter in a valid name (52 letters (uppercase and lowercase), 10 number and _), 64 if we consider `null` characters used for padding names shorter than 16 characters. By using this fact, each possible letter can be mapped to its index, and unique ids can be calculated incrementally.

- **Advantages:** Flexibility in case the maximum length for names is increased in the future.
- **Disadvantages:** No flexibility in case new valid characters are added in the future.

Start by reviewing issue #45 and the existing cracked-account UUID generation described here, including the MD5-based OfflinePlayer: input. Compare the two proposed encodings and determine which requirements the mapping must satisfy. Done means an agreed bidirectional UUID mapping design is selected and its compatibility constraints are documented.

Written by the indexing model from the issue text.