use pImpl idiom for gamemap in game_board, avoiding map.h include

unit_map is modified in many places that don't normally include
map.hpp, so to avoid doing harm by adding this class, I am making
it hold a scoped pointer to the gamemap instead of an actual
gamemap. This results in very few changes.

- The implementation of game_board has to use the pointer.
- game_board must use copy and swap idiom
- lua get terrain function must pass strings to the game_board,
  not enums, and we parse them as std::strings in game_board
  instead of as c strings in lua api (which was a bit gross anyhow)