Implement std::error::Error for all public error types

[theduke]

• GraphQLError
• ParseError
• LexerError
• RuleError
• FieldError

[davidpdrsn]

I looked into this yesterday and it isn't totally straight forward. std::error::Error requires a Display implementation which Spanning<ParseError<'a>> and RuleError doesn't have. They should be possible to add but I wasn't sure what the most helpful implementations might be.

I might just make some initial implementation so we have something to discuss.

[theduke]

All errors should get a Error impl, it's a big oversight that they didn't have it yet.

The only limitation here is that we can't include surrounding input query lines in the error message but that's fine I think.