dimension. The advantage is that evaluating a dynamic-sized block in a fixed-size
matrix no longer causes a dynamic memory allocation. Other new thing:
IntAtRunTimeIfDynamic allows storing an integer at zero cost if it is known at
compile time.
column-major order, even if storage is row-major. Benchmark showed that adapting
the traversal order to the storage order brought no benefit.
Also do some cleanup after Gael's big patch.
