[Nonlinear] make _(Function|Subexpression)Storage immutable

[odow]

Follow up to #2620 that brings things down to

julia> @benchmark(
           MOI.Nonlinear.ReverseAD._reverse_mode($(evaluator.backend), x),
           setup=(x=rand(num_variables(model))),
       )
BenchmarkTools.Trial: 50 samples with 1 evaluation per sample.
 Range (min … max):   97.891 ms … 117.640 ms  ┊ GC (min … max): 0.00% … 0.00%
 Time  (median):      98.921 ms               ┊ GC (median):    0.00%
 Time  (mean ± σ):   100.545 ms ±   3.878 ms  ┊ GC (mean ± σ):  0.00% ± 0.00%

  █ ▁▂                                                           
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  97.9 ms          Histogram: frequency by time          118 ms <

 Memory estimate: 0 bytes, allocs estimate: 0.

I can't find the reason I made these mutable. See the latest commit. I guess I wanted to make minimal changes to the existing code.

They didn't exist in the original JuMP code:
https://github.com/jump-dev/JuMP.jl/blob/0a6b1e426425e69234f073079c3ebe6301a43f25/src/_Derivatives/forward.jl#L30-L31
so when I created them in #1803 I guess I had a reason for making them mutable at some point that didn't survive the final factoring.