Initial cGridStructGen refactoring.

Ref.: #987.
Initial cPrefabPiecePool refactoring.

Ref.: #986.
cNetherFortGen uses cGridStructGen.

WIP, this doesn't work properly yet.
Fixed MSVC 64-bit build warnings.
Merge branch 'master' into GridStructGen
Fixed cGridStructGen.

Now cNetherFortGen works with the new architecture.
Mineshafts generator rewritten to use GridStructGen.
cWormNestCaves rewritten using cGridStructGen.

Ref.: #987.
Ravines rewritten using cGridStructGen.

Ref.: #987.
Removed an unused macro.
Removed an unused NetherFortGen variable.
Removed unused constants from MineShafts and Ravines.
Merged branch 'master' into GridStructGen.
Merge pull request #993 from mc-server/GridStructGen

Grid struct gen