Add macro bindings for views

[joshua-spacetime]

Description of Changes

Adds the #[view] procedural macro and module describers for views.

#[view(public)]
fn player(ctx: &ViewContext) -> Vec<Player> {
    ctx.db.player().identity().find(ctx.sender).into_iter().collect()
}

#[view(public)]
fn player(ctx: &AnonymousViewContext, level: u32) -> Vec<Player> {
    ctx.db.player().level().filter(level).collect()
}

Note, this deviates from the proposal in that views may only return Vec<T> or Option<T>. They can't return an arbitrary SpacetimeType.

API and ABI breaking changes

None

Expected complexity level and risk

2

Testing

• Negative compile tests
• Negative publish (module validation) tests
• Test system tables are updated accurately

[Centril]

I think this is worth mentioning in the PR description. In some sense it is breaking, but not in a way we promise.

[joshua-spacetime]

In what sense exactly?

[Centril]

If this were to be implemented manually, we would break such users. We haven't promised such stability, but it might be worth mentioning nonetheless.

[Centril]

Maybe an example of a join and an example of a part of a table?