Added some ideas for new unit abilities.
Attempt #1 at improving AI performance.

Storing units in the game state naively by entity ID led to a large
amount of wasted space.  Lookups were fast but the AI had to copy a 100+
element vector with only a dozen active units in it.  This first attempt
uses a hash table instead and seems to be faster.
Trying out delegating constructor.
Further improve AI performance.

Replaced the unordered_map in the previous change with a sorted vector.
This further reduced the amount of work needed to copy a GameState
object (one of the main drivers of minimax performance).
Draw after 3 full rounds of no units being killed.