Changes:
* Introduce default enabled nix feature flag to allow for non nix usage (#41)
* Update minimum supported Rust version to v1.60

Fixes:
* minor clippy fixes
* update references to IETF documents
* Fix typos
* Update Dockerfile.build (#31)
* Fix and simplify Dockerfiles

Updates:
* update nix to 0.26 and fix deprecation warnings (#46)
* update signal-hook requirement from =0.1.8 to =0.3.15
* update cbindgen requirement from 0.21 to 0.24
* update vsock requirement from 0.2 to 0.3