Rust 2021, updated `rand` dep, canonicalized `use_std` to `std`, file names

[virtualritz]

• Updated Rust edition to 2021 and made resp. req. changes (use stuff, compiler feature).
• Renamed the use_std feature to std; this is what popular crates commonly use nowadays.
• Updated rand dev-dependency to latest and made resp. req. changes.
• Renamed README.md and LICENSE*.md` file(s) to uppercase (default on GH).
• Fixed license string in Cargo.toml.
• Removed deprecated feature from rustfmt.toml.
• Ran rustfmt.