Update most `~str` things to `StrBuf`.
Update for Result-yielding Decodable::decode().
Rename crate roots to lib.rs (convention).
Use new inner attribute syntax.
Update for struct publicity swap.

(Used to be public by default, private when marked explicitly; now it's
private by default, public when marked explicitly.)
Update rust-phf
Update macros crate for new Rust
Update tests and benchmarks for new Rust
Note Rust version in README