Cache fusionblocks together with the tree positions of each block #587

For non-abelian sectors, fusionblocks(W) is usually what is needed, but only fusiontrees(W) is cached:

• fusionblocks(W) regroups and sorts the cached trees on every call.
• GenericTreeTransformer then looks up the position of every tree of every block (gettoken).

Proposal: cache, per HomSpace sector structure (like sectorstructure), the blocks and the positions of their trees in fusiontrees(W).

Open points:

• Separate cache, or a field of SectorStructure.
• Conjugated sources: blocks of Vsrc', positions in fusiontrees(Vsrc).
• Destination positions from the cache of Vdst: one lookup per block instead of per tree.
• Build on Replace TensorKit cache machinery with MemoizationKit.jl #568 (MemoizationKit caches).

Gain: this bookkeeping is serial, and its share of building the transformers grows as the recoupling itself gets cheaper (#580, #581).