v0.21.0

Added

• Support for OTP22.
• Rust linting with clippy.
• Support for decoding IOLists as binaries, Term::decode_as_binary.

Changes

• rustler_codegen is now reexported by the rustler crate. Depending on the rustler_codegen crate is deprecated.
• erlang_nif-sys has been renamed to rustler_sys and vendored into the rustler repo.
• Replaced the hand-rolled TOML parser in rustler_mix with the toml-elixir package.
• Improve error messages for derived encoders/decoders.
• Rust bool now corresponds only to booleans (false, true) in Elixir. Previously, nil and false were both decodable to
  bool. To use the previous behaviour, a Truthy newtype was introduced.