Validator list

[kylegranger]

This code (my very first rust code ever) basically used this rippled function -- buildValidatorList() -- as a model, to see what steps are used to create and sign a manifest, as well as a validator blob. https://github.com/XRPLF/rippled/blob/master/src/test/overlay/compression_test.cpp#L316

The verification side does the following:

• receives a manifest
• extracts the signatures
• recreates the manifest without the signatures
• appends a 4-char prefix to it,
• gets a 32-byte digest from SHA512 (throwing away half of those 64 bytes),
• then does the signature verification, once for each of three different signature (matching each sign() call in the C++ code), also with the validator blob data.

Our keys are generated before hand, one key pair for the master key, and one for the signing key. The master public key has been added in validators.txt. The readme has been update to reflect this new content.

[kylegranger]

last of changes have been committed, including:

• serialization uses bytes::BytesMut
• all magic numbers moved to consts
• validators.txt file added to project, in setup folder
• readme updated, for copying this file to ~./ziggurat/...
• use &str over String, pass &[T] over &Vec
• shorten sleep from 5 min to 5 secs
• test renamed, updated SPEC.md, added TODO for real test verification.
• use string literal for 4-char u32
• removed unused code, moved expiration creation to own function

[kylegranger]

This should be pretty solid now. Last round of changes included:

• refactored creating manifest, signing, appending prefix, signing buffers
• renamed many, many variables.
• restored text in SPEC.md mistakenly deleted (merge error)