[Rust] Support macro repetitions inside structs

[FichteFoll]

This only addresses a specific case of macro repetitions inside the struct body and similar changes to other statements with body blocks are likely also necessary, but I'm keeping the change to a limited scope in order to observe its effects first. Further reports of such issues can be handled analogously.

Closes #3904.

[FichteFoll]

The other reason I didn't do it for other block statements is that I didn't have any (valid) examples at hand to verify with and I'm not comfortable enough with macros to craft them ad-hoc.

[FichteFoll]

This doesn't handle macros like println!($($arg)); or println!($($arg)?); well and I don't really see how it could … Putting the PR on hold for now.

[jrappen]

... putting the PR on hold.

@FichteFoll Do you plan to pick this up again?

[FichteFoll]

Not in the near future. I don't think this can be handled cleanly with the current structure and inside a prototype match. A better approach would likely be to either create a derived syntax with macro block support added in every relevant context that a macro pattern can be used, or directly add support for macro patterns into each syntax construct natively with the upside of only having one syntax to maintain but the downside of this single syntax becoming noticably more complex.

Haven't really thought about it more than that, however.