flexible in that works for symbols or strings

[oxinabox]

like in #3 we want a version of in that lets accepts both strings and symbols.

I don't want to call it in as that is too much type piracy #4
so flexible_in or something.
converting_in

[oxinabox]

if we wanted to make this more general than just strings and symbols then
having JuliaLang/julia#35102
would simplify this a bit, but can work without it.

[oxinabox]

@nickrobinson251 says this is a bad idea in the first place, and one should know if one is dealing with symbols or strings.

I am inclinded to agree