Add support for optional trait functions in method calls

This adds compilation support for optional trait functions with a function
block. This does not support constants or default types yet.

Trait references must do type resolution of their block, but this is only
performed once a trait is resolved. In order to avoid recursive trait
resolution we use a on_resolved callback to invoke this to occur. The trait
resolver works as a query based system so you have to avoid the case of
recursively resolving the trait untill it is stored in the internal
mappings.

Addresses #542

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