Performance: Avoid re-computing tree hashes

[ranocha]

Code such as

BSeries.jl/src/BSeries.jl

Line 775 in 8bb9b95

series[t] = series_integrator[t]

or

BSeries.jl/src/BSeries.jl

Line 956 in 8bb9b95

series[t] += series_ex[t] - substitute(series, series_integrator, t)

basically computes the hash of the tree t twice. Sadly, there is no stable Base API functionality to avoid this, see https://discourse.julialang.org/t/document-export-ht-keyindex-from-dict-jl/83738 and JuliaLang/julia#33758.
It may be worth checking https://github.com/andyferris/Dictionaries.jl as an alternative.