Commit 23b0c12d introduced caching in chain
rule derivation (used by block decomposition), which increased speed for mfs >
0, but actually decreased it for mfs=0.

This patch introduces the pre-computation of derivatives which are known to be
zero using symbolic a priori (similarly to what is done in the non-chain rule
context). The algorithms are now identical between the two contexts (both
symbolic a priori + caching), the difference being that in the chain rule
context, the symbolic a priori and the cache are not stored within the ExprNode
class, since they depend on the list of recursive variables.

This patch brings a significant performant improvement for all values of the
“mfs” option (the improvement is greater for small values of “mfs”).