New build_lab, other underground overmap code cleanup

[sparr]

This patch consolidates build_ice_lab and build_lab, and then changes the generation algorithm. The new algorithm produces less boxy more "organic" lab shapes. I tweaked the parameters so that the average total size of the lab is still about the same.

One perk of this change is that there will be more choke points in labs, so the lab mapgen could now be updated to put more turrets / traps / strong zombies at the choke points and less in random rooms. This will reduce the "turret guarding nothing, and I can just walk around it" problem that has been lamented on the forum.

also some code cleanup and styling in various places

[sparr]

This gist includes some sample layouts for the old and new generators: https://gist.github.com/sparr/2e6f014d45b5e44c34e7

[DeadLeaves]

While I have no issue with it, wont this make lab challenge really difficult?

[sparr]

@DeadLeaves changing mapgen_lab to put turrets in choke points might make the lab challenge more difficult, but that's not what this PR does. this one just rearranges the overall shape of the lab, without changing anything about what goes in the various rooms.