Function mapcan

pub(crate) fn mapcan<'ob>(
    function: &Rt<Slot<Gc<FunctionType<'_>>>>,
    sequence: &Rt<Slot<Gc<ObjectType<'_>>>>,
    env: &mut Rt<Env<'_>>,
    cx: &'ob mut Context<'_>,
) -> Result<Gc<ObjectType<'ob>>>