Minor Changes

This release is relatively uninteresting, with a few internal changes:

• Switch away from deprecated functions in the standard library.
• Add a few more unit tests - we now have 100% test coverage.
• Fix typos and a dead link in README.md.
• Explicitly forbid unsafe code via #![forbid(unsafe_code)]

Implement additional traits on error types

• Implement additional traits for error types (contributed by GKnirps)
• Fix typo in README.md (contributed by ayakovlenko)
• Other minor tweaks to the README and documentation

GTIN Validate v1.1.0

• Minor performance improvements all around.
• Significant speedups when the length of GTIN codes is incorrect.
• Benchmark suite included to measure performance.
• Cleaner, more idiomatic code.