Work with symbols and integers on the RHS

```
julia> df = DataFrame(a = [1, 2]);

julia> x = [true, false];

julia> @subset df x
ERROR: LoadError: MethodError: no method matching fun_to_vec(::Symbol; no_dest=true, outer_flags=(var"@byrow" = Base.RefValue{Bool}(false), var"@passmissing" = Base.RefValue{Bool}(false), var"@astable" = Base.RefValue{Bool}(false)))
```

Need to write a method of `fun_to_vec` for symbols and integers. 