Performance and Rust idiomatic improvements

[maciejhirsz]

• Removed bitvec dependency, wrote a quick BitWriter struct that allows pushing 11 bits onto a Vec<u8> at once.
• Removed a bunch of unnecessary allocations. This results in some (not all) functions that used to take generic S: Into<String> to take &str, which is more idiomatic although it is a breaking change on the API surface. Functions that need allocated strings (constructors that return owned structs mostly) remained unaffected.
• wordlist and wordmap work using &'static str instead of Strings to avoid unnecessary allocations (not very critical).
• Replaced Result<T, Error> with custom Result<T> from error_chain.
• Bunch of smaller idiomatic changes that I likely can't recall here.
• Added benchmarks (rustup run nightly cargo bench). Phrase validation time has been cut to nearly a quarter of original time.

[maciejhirsz]

Closing this in favor of another, more extensive PR.