[serde] `Deserialize`+`Serialize` impl for (this crate's) `T: Symbol` implementors

[fogti]

Symbols already get verified when used via resolve, so it should be no problem to provide appropriate serde::Deserialize and serde::Serialize implementations for symbols, such that the symbols and the interner can be easily serialized as part of an overall container structure.

[bonsairobo]

Looks like a pretty easy PR. @Robbepop Do you think we could merge these changes soon?

[abs0luty]

It looks like repository is a bit abandoned :\

[bonsairobo]

@abs0luty Yea I've since started using the lasso crate.

[Robbepop]

The repository is not avandoned officially but I am currently not having the time and energy to have focus for it.

[bonsairobo]

@Robbepop Hey no worries. Thanks for your contributions. I wish you the best.

[abs0luty]

@Robbepop Hey that's fine, thanks you for all the stuff you've already made. I also wish you the best.