Fix #484, support multiple message parts in assert

[WebFreak001]

Also modified the AST for regular asserts, for future proofing and easier implementation.

cc @RazvanN7

Introduces a deprecation in opEquals, it's fixed in #485

[RazvanN7]

Thanks!

[RazvanN7]

Could also please push a new tag? That will unblock phobos PRs using this feature.