Complete, but broken, implementation.

Present issue appears to be that when the upper and lower boundaries
are the same, there is no room for the table to propogate, so I need
to ensure the U,D,L,R constraints of a path allow for unit width
when the path is horizontal or vertical (diagonal is OK).

Fails on sample.in test #2.
Works with all official examples.

Major change is that the entire backwards-reconstructed path
found in each subproblem is included in the U,D,L,R limits implied
by the path, such that if 2 subproblems fall the same subpath,
any problem between them will have at least 1 unit wide of table
path that will carve along the border, rather than get squeezed
out. Examples paths and U,D,L,R limits for them are given in new
spreadsheet CS161_Path_Limits_Gold_Standard_Single_01.

There are several other minor fixes and debug code changed, mainly
dealing with using Aext instead of A, or the wrong size array
for the limits.