Support for custom scalars

[CodesInChaos]

I'm interested in proper support for custom scalar types. This has some overlap with the enum support mentioned in the readme, in particular this applies to them as well:

However end users probably don't want multiple enums for each generated query module as this would break sharing. To get around this, there should be a macro for generating the shared types used by all other.

There is one additional challenge however: Custom scalars usually have constraints, so the new function needs to contain validation and return a custom Result type. Auto generating that isn't trivial for standard constraints, and practically impossible for expression constraints. So there needs to be a design that allows mixing auto-generated code with custom written code.

[ifiokjr]

I think I'll leave the custom scalar types until the edgedb-protocol library is upgraded to protocol version 2.0. This provides names for the scalar types which can be added to the shared types.

Regarding constraints, I was thinking there might be a way to provide the https://github.com/Keats/validator as an option. But I can't think of a good API for this.

It's not a high priority for my current project, but if you can think of a compelling way to integrate this, I'm happy to accept contributions.