warn on overloaded `=` with `refc`

[arnetheduck]

What happened?

Per conversation with @Araq, overloading = is broken in the refc GC and cannot work with the sequence implementation - there are also bugs with reference counting in the generated code which lead to hard-to-debug bugs when / if = leaks into the codebase accidentally.

Calling an overloaded = should thus give an error at compile-time when used with refc since there is no case where one would actually want them.

Nuance: there is one use case for overloaded = which maybe is not broken: when using it to disable copying altogether for a type, ie when declaring it without a definition - thus it makes sense to make the compile-time error happen when = would be called, not when it's declared.

Nim Version

1.6.x

Current Standard Output Logs

No response

Expected Standard Output Logs

No response

Possible Solution

No response

Additional Information

No response

[bung87]

overloading = is a case for cpp, see #13093