post-incr-rs

Just add traits for post increment and decrement.

```rust use postincr::PostIncr as _; use postincr::PostDecr as _;

let mut x: i32 = 0; asserteq!(x.postincr(), 0); asserteq!(x.postincr(), 1); asserteq!(x.postincr(), 2);

asserteq!(x.postdecr(), 3); asserteq!(x.postdecr(), 2); asserteq!(x.postdecr(), 1); ```