Trinary — Rust types for trinary logic

Based on TriState code base.

A three-valued type equivalent to Option<bool>:

rust enum TriState { Yes, No, Unknown }

A nice way to use this type is with a domain-specific type alias via pub use. (For esoteric reasons, a simple typedef-style type alias doesn't work, though this Rust limitation will eventually be removed.) For example, a spam classifier:

```rust extern crate tristate;

pub use tristate::TriState as Spam;

trait Classify { fn classify(&self) -> Spam; }

impl Classify for Message { /* ... */ }

// ...

match message.classify() { Spam::Yes => /* ... /, Spam::No => / ... /, Spam::Unknown => / ... */ } ```