add serde feature and implement Serialize and Deserialize on most error types

[Arne91]

To serialize and deserialize the ParseError, the crate "serde" is added.

[felixwrt]

Thanks for taking the time to contribute. I'm ok with adding serde implementations to the code base, but they should be optional. Could you change that, please? There are lots of crates that have an optional serde feature, so it shouldn't be difficult to find inspiration. Other than that, please also add the new feature to the crate's documentation. Thanks in advance!

[Arne91]

Thanks for taking the time to contribute. I'm ok with adding serde implementations to the code base, but they should be optional. Could you change that, please? There are lots of crates that have an optional serde feature, so it shouldn't be difficult to find inspiration. Other than that, please also add the new feature to the crate's documentation. Thanks in advance!

Thank you for your quick feedback. That is a good point.
I added it as optional "serde" feature. I also added it in the crate docs.

[felixwrt]

I'm sorry that it's been taking me so long to integrate this - I've now finally managed to do so. Thanks again for contributing and let me know if there's more you'd like to see changed.