3055: Fix compile error when trying to use static slot methods, fixes: #3039 r=davidhewitt a=willstott101
Whilst having static slot methods is of dubious value in the first place, there's little reason not to support it by bringing the different function call macros more in-line with each other.
I had no idea which test file to use so I just made a new one... `test_methods.rs` has no slots in and the test files that do use slots seem to be specific to their protocol. Let me know if you'd like the test somewhere else.
Thanks!
Co-authored-by: Will Stott <willstott101@gmail.com>
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