Add compliance tests

[epage]

https://github.com/BurntSushi/toml-test is the semi-official compliance test suite. toml_test_harness is a rust crate that makes it semi-easy to add it to cargo test

Example

• https://github.com/toml-rs/toml/blob/main/crates/toml/tests/decoder_compliance.rs
• https://github.com/toml-rs/toml/blob/main/crates/toml/tests/decoder.rs
• https://github.com/toml-rs/toml/blob/main/crates/toml/tests/encoder_compliance.rs
• https://github.com/toml-rs/toml/blob/main/crates/toml/tests/encoder.rs

However, I'm unsure how well this can be integrated without a basic_toml::Table type (which is where I stopped in preparing my contribution). If toml::Table is compatible enough for datetimes, then maybe that can work.

[epage]

FYI I added basic-toml to a TOML test matrix which includes compliance testing

You can find the relevant compliance testing code at https://github.com/arp242/toml-test-matrix/tree/master/scripts/rust-basic-toml

As of right now, there are 21 decoder failures and the decoder failures are preventing verifying the encoder, so the number of encoder failures is unknown. There are also 4 invalid cases that are accepted. See https://arp242.github.io/toml-test-matrix/output/rust-basic-toml.html