refactor: fix type instability within get_constants

[MilesCranmer]

Because Ref{T} is not the same as Base.RefValue{T}. 🫠

[github-actions]

Benchmark Results

	master	cdb63b0...	master/cdb63b0eaf1435...
eval/ComplexF32/evaluation	7.39 ± 0.5 ms	7.38 ± 0.45 ms	1
eval/ComplexF64/evaluation	9.7 ± 0.75 ms	9.67 ± 0.69 ms	1
eval/Float32/derivative	10.7 ± 1.5 ms	10.5 ± 1.4 ms	1.01
eval/Float32/derivative_turbo	10.7 ± 1.5 ms	10.5 ± 1.5 ms	1.01
eval/Float32/evaluation	2.7 ± 0.22 ms	2.7 ± 0.22 ms	1
eval/Float32/evaluation_bumper	0.525 ± 0.013 ms	0.57 ± 0.012 ms	0.923
eval/Float32/evaluation_turbo	0.684 ± 0.025 ms	0.693 ± 0.027 ms	0.986
eval/Float32/evaluation_turbo_bumper	0.524 ± 0.013 ms	0.571 ± 0.013 ms	0.918
eval/Float64/derivative	14.2 ± 0.7 ms	13.4 ± 0.54 ms	1.06
eval/Float64/derivative_turbo	14.5 ± 0.81 ms	13.3 ± 0.52 ms	1.09
eval/Float64/evaluation	2.88 ± 0.25 ms	2.91 ± 0.24 ms	0.99
eval/Float64/evaluation_bumper	1.2 ± 0.042 ms	1.26 ± 0.043 ms	0.952
eval/Float64/evaluation_turbo	1.16 ± 0.058 ms	1.17 ± 0.058 ms	0.997
eval/Float64/evaluation_turbo_bumper	1.19 ± 0.042 ms	1.26 ± 0.043 ms	0.948
utils/combine_operators/break_sharing	0.0407 ± 0.0014 ms	0.041 ± 0.0013 ms	0.992
utils/convert/break_sharing	28.4 ± 1.1 μs	28.6 ± 1 μs	0.992
utils/convert/preserve_sharing	0.13 ± 0.0034 ms	0.129 ± 0.0034 ms	1.01
utils/copy/break_sharing	29 ± 0.83 μs	29.6 ± 0.92 μs	0.979
utils/copy/preserve_sharing	0.127 ± 0.0022 ms	0.128 ± 0.0027 ms	0.989
utils/count_constants/break_sharing	11.3 ± 0.18 μs	10.8 ± 0.16 μs	1.05
utils/count_constants/preserve_sharing	0.111 ± 0.0021 ms	0.112 ± 0.0024 ms	0.996
utils/count_depth/break_sharing	17.6 ± 0.37 μs	17.8 ± 0.38 μs	0.987
utils/count_nodes/break_sharing	10.1 ± 0.16 μs	10.3 ± 0.17 μs	0.98
utils/count_nodes/preserve_sharing	0.114 ± 0.0022 ms	0.114 ± 0.0025 ms	0.996
utils/get_set_constants!/break_sharing	0.122 ± 0.0041 ms	0.0424 ± 0.00086 ms	2.87
utils/get_set_constants!/preserve_sharing	0.314 ± 0.0059 ms	0.231 ± 0.0057 ms	1.36
utils/has_constants/break_sharing	4.3 ± 0.21 μs	4.32 ± 0.21 μs	0.995
utils/has_operators/break_sharing	1.78 ± 0.015 μs	1.79 ± 0.022 μs	0.998
utils/hash/break_sharing	0.0326 ± 0.00045 ms	0.0325 ± 0.00044 ms	1
utils/hash/preserve_sharing	0.134 ± 0.0021 ms	0.135 ± 0.0026 ms	0.995
utils/index_constants/break_sharing	27.3 ± 0.67 μs	27.9 ± 0.77 μs	0.979
utils/index_constants/preserve_sharing	0.127 ± 0.0025 ms	0.127 ± 0.0029 ms	1
utils/is_constant/break_sharing	4.76 ± 0.22 μs	4.79 ± 0.22 μs	0.993
utils/simplify_tree/break_sharing	0.153 ± 0.015 ms	0.153 ± 0.015 ms	0.998
utils/simplify_tree/preserve_sharing	0.27 ± 0.017 ms	0.268 ± 0.017 ms	1.01
utils/string_tree/break_sharing	0.386 ± 0.016 ms	0.384 ± 0.013 ms	1.01
utils/string_tree/preserve_sharing	0.524 ± 0.027 ms	0.532 ± 0.02 ms	0.985
time_to_load	0.228 ± 0.0038 s	0.226 ± 0.0023 s	1.01