Fail-safe file sequence

Implemented in Rust. Inspired by this Java implementation

Usage

```rust let initialvalue = 1; let seq = FileSeq::new(storedir, initial_value).unwrap();

// Get current value seq.value().unwrap();

// Increment by 1 and get seq.incrementandget(1).unwrap();

// Get, then increment by 1 seq.getandincrement(1).unwrap(); ```

Changelog

0.2.0 (2020-09-07)

• Ignore errors on FileSeq::delete function #1