Procedural macro 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, anonymous)]
fn player(ctx: &AnonymousViewContext, level: u32) -> Vec<Player> {
    ctx.db.player().level().filter(level).collect()
}

TODO:

1. Support for T and Option<T> return types
2. System table updates and module validation on publish

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