void object fields are now ignored by codegen and fields/fieldPairs iterator

[nc-x]

Closes #3734

[nc-x]

Fails due to 1fb5dd2

[Araq]

The void type for object fields is allowed though and the codegens should ignore these fields. I think.

[nc-x]

cc @Araq

[Araq]

Thanks but your change in system.nim made me reconsider. It's yet another special case that gets exposed to every piece of code that uses field iteration. --> Better not allow it and require code to use the more explicit when T isnot void variant for the outlined use case "Table[K, void] is Set[K]".

Alternatively the fields iterators should ignore void fields automatically.

[nc-x]

Alternatively the fields iterators should ignore void fields automatically.

Done.

[nc-x]

cc @Araq

[krux02]

interesting that void object fields are even allowed. But apart from that, I am ok with these changes.

[Araq]

You should use isEmptyType instead of testing for tyVoid directly.

[nc-x]

Done.