Consider testing Function Key mappings before mapping

[fourjay]

I just realized that terminus was stepping on high function key mappings (<F21>-<F24>). My personal mappings were broken (I'd just assumed vim had a lower limit of "virtual" function keys then I'd expected). I realized my mappings were broken, as I should have checked if the high mappings were used by something else. It seems useful to check mappings in some fashion in terminus.