Add serde support to AST structs and enums

[panarch]

Apply serde to AST structs and enums to be serializable/deserializable.
serde support is optional, can be activated by feature named "serde".

It resolves the issue https://github.com/andygrove/sqlparser-rs/issues/183

I applied to all enums and structs which has #[derive(Debug, Clone, PartialEq, Eq, Hash)] in ast directory.

[coveralls]

Pull Request Test Coverage Report for Build 130824818

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 91.716%

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Totals
Change from base Build 130660351:	0.0%
Covered Lines:	4063
Relevant Lines:	4430

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💛 - Coveralls

[nickolay]

Excellent, thanks! I think it might be a good idea to add a note about this in README and/or update the cli example to output this instead of Debug representation when available, but this can be done separately.

[panarch]

@nickolay wow, really fast, thanks!
It also sounds good to add code example, I'll make a new PR for that.