Investigate improving load/compile times by changing optimization level

[mtfishman]

It looks like Plots.jl improved load times and compile times by using a new Julia v1.5 feature that can change the optimization level for a specified module (see here and here). We should try this in ITensors, when v1.5 is released. It may work well, since performance critical code is in other modules like NDTensors and LinearAlgebra, so it may not affect performance but will hopefully improve load times and compile times.