`nameof(f)` not defined for all functors

[baggepinnen]

https://github.com/JuliaSymbolics/SymbolicUtils.jl/blob/7534749988566b76940b60b462f3fdb72d109c59/src/types.jl#L677

SymbolicUtils often calls nameof on functions, but when a callable struct appears as the argument to nameof, this fails.