feat: simple type support

[beltram]

Hi,

I've crated this PR to add support for CBOR Simple types as defined in RFC 8949 Section 3.3.

I've roughly used the same approach you used for tags. I chose to fail in case a reserved simple type was used (0..=19) or (24..=31) and I think this is what causes the fuzzer to fail. He also seems not to differentiate one-byte and two-byte variants of simple types which causes some other errors in the fuzzer. I would like some pointers from your side to understand what happens under the hood and how to fix that.

Additionally, I wanted to take advantage of this PR to introduce a new variant Value::Undefined instead of Value::Simple(23). I think that's fine since the enum in #[non_exhaustive]. Let me know if that sounds reasonable to you.

Any help getting this PR fixed and merged will be very appreciated :)

Thank you

Closes #60