Refactor

[maciejhirsz]

Disclaimer: This is a really big PR and I'm happy to make a fork of the crate and maintain it on crates.io if you'd rather not change the API as much as I've done here.

• Reworked most of the API surface, removed methods dealing with hex-encoded strings, added LowerHex and UpperHex trait implementations to both Mnemonic and Seed for printing those out as hex. In general, I don't think parsing hex-encoded strings is within the scope of the library.
• Added remaining languages.
• wordlist now operates on &'static str instead of allocating Strings. When retrieved from Language, it returns a &'static [&'static str] instead of a &'static Vec<String>.
• Similarly, WordMap is a newtype wrapping around FxHashMap<&'static str, u16> (which is just a regular HashMap using fast hashing algo from rustc, it's not as secure in terms of collisions as the default, but since we aren't allowing any insertions it shouldn't matter).
• Replaced Result<T, Error> with custom Result<T> and switched to using the bail! macro from error_chain to reduce some boiler plate.
• Removed bit-vec, bitreader and data-encoding dependencies.
• Added benchmarks (rustup run nightly cargo bench) and a bunch new tests when necessary.

Performance:

• Validating phrases is now ~6x faster.
• Creating a new Mnemonic from entropy is way faster (I don't have the original figures) due to reduced number of allocations (worst case scenario is 2-4 depending on the length of the key).
• Generating a new Mnemonic (skipping Seed generation) from random entropy is now 20x+ faster.

test from_entropy ... bench:         418 ns/iter (+/- 229)
test new_mnemonic ... bench:         560 ns/iter (+/- 347)
test new_seed     ... bench:   1,404,971 ns/iter (+/- 397,740)
test validate     ... bench:         657 ns/iter (+/- 591)

Generating a Seed is obviously most expensive due to the 2048 pbkdf2 rounds.

[maciejhirsz]

Closing since it's included in #15.