`fusionblockstructure` should reuse data that is degeneracy-independent

[lkdvos]

Within the cached data for fusionblockstructure, there is data that depends on the sectors only, and data that depends on the sectors and degeneracies.
Decoupling these should decrease the memory pressure and increase the overall speed of the package, since we could avoid recomputing fusiontrees in workflows where the degeneracies are often fluctuating slightly, but the charges are somewhat fixed.