Added checks ensuring submatrix matches expectation of framing matrix.

Inlined fillN into its one call site.
Refactored matrix building, and added a specialized path for dims == 1.
Test checking that dims == 1 is probably consistent with dims > 1.
Added implementations for fill{2,3}_{leaf,subm}.
Coming closer to plausible parallel-ized ParallelMatrix.

Still some clear bugs e.g. on inputs like:

  new ParallelMatrix([30,30], [2], function (i,j) { return new ParallelMatrix([2], function (k) { var l=0; var i_ = i+1; j_=j+1; k++; l++; return i_*1000+j_*100+k*10+l; }); })

which somehow has a bunch of undefined's mixed in.