* error return tracing is disabled in release-fast mode
* add @errorReturnTrace
* zig build API changes build return type from `void` to `%void`
* allow `void`, `noreturn`, and `u8` from main. closes#535
...field to const slice parameter
we use a packed struct internally to represent a const array
of disparate union values, and needed to update the internal
getelementptr instruction to recognize that.
closes#664