gift: use built-in benchmarks instead of Criterion (#436)

Criterion is a relatively big dependency which unnecessarily gets pulled
during testing. Additionally, it results in the [security
warnings](https://github.com/RustCrypto/block-ciphers/security/dependabot/1).

In addition to the benchmark changes, this PR also slightly tweaks
module structure of the crate.