Renamed Plan_alloc_id() --> alloc_id() and moved it to level.lua
Moved Plan_choose_liquid() and Plan_choose_darkness() --> level.lua
(and also some code setting up LEVEL fields).
Removed the (now empty) planner.lua script file.
Removed 'prefab.lua' script file (the obsolete code-based prefab stuff)
1. Renamed WAD prefab code from 'wad_fab.lua' --> 'prefab.lua'
2. Moved all require statements --> oblige.lua
Began work on new Stairwell logic.  We will detect certain shapes of small
hallways (and certain entry/exit locations).  This commit mainly has the
STAIRWELL_SHAPES table with all the shapes <= 2x2 seeds.
STAIRWELL_SHAPES : added the 2x3 and 3x3 shapes, and gave each shape a name
(like A1, C3, L2) which will help when debugging this guff.
Stairwells : added code to test all the shapes.
Stairwell shapes : added 'straight' field to non-curved stairwells, and
removed some of the mirrored variants.
Added skeletal function for building stairwells : Layout_build_stairwell()
Partial work on tracing edge-loops for areas (esp. the outer loop)
Edge loops : fleshed out algorithm to trace the outer loop.
Fixed a few bugs in new edge tracing code.