Region & System edge population

[yakra]

• Restore bool operator () (const item value) const {return (*this)[value-start];} as there's finally a use for it.
  (Added in 51d5e88 & removed in ed33184 in the g_tmb branch.)
• Checking whether edges are already contained in the set will save us some pointer-chasing, and more significantly for system sets, iterating through concurrent segments.
• PlaceRadius::ve_search too. Saves us from getting v2 and doing a load of trig on it.