Allow operator aliases

[MilesCranmer]

This creates the empty_old_operators keyword to @extend_operators which effectively lets you prevent the call to empty!(LATEST_BINARY_OPERATOR_MAPPING), which means you can have multiple operators producing the same index. This is good for aliases like safe_pow and ^ meaning the same thing.

[coveralls]

Pull Request Test Coverage Report for Build 5771392058

• 18 of 20 (90.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.08%) to 91.017%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/OperatorEnumConstruction.jl	18	20	90.0%
<!--	Total:	18	20	90.0%	-->

Totals
Change from base Build 5734174311:	0.08%
Covered Lines:	1074
Relevant Lines:	1180

---

💛 - Coveralls

[github-actions[bot]]

Benchmark Results

	master	580ed0fb0ca324...	t[master]/t[580ed0fb0ca324...]
eval/ComplexF32/evaluation	11.1 ± 0.68 ms	11.1 ± 0.68 ms	1
eval/ComplexF64/evaluation	13.7 ± 0.87 ms	13.5 ± 0.96 ms	1.01
eval/Float32/derivative	17.4 ± 2.2 ms	16.7 ± 2.6 ms	1.04
eval/Float32/derivative_turbo	19.4 ± 2.2 ms	19.5 ± 2.5 ms	0.995
eval/Float32/evaluation	3.64 ± 0.28 ms	3.65 ± 0.3 ms	0.996
eval/Float32/evaluation_turbo	1.17 ± 0.062 ms	1.15 ± 0.06 ms	1.01
eval/Float64/derivative	20.3 ± 0.85 ms	20.6 ± 0.73 ms	0.985
eval/Float64/derivative_turbo	24.1 ± 0.72 ms	24 ± 0.73 ms	1
eval/Float64/evaluation	3.72 ± 0.31 ms	3.69 ± 0.28 ms	1.01
eval/Float64/evaluation_turbo	1.63 ± 0.077 ms	1.63 ± 0.082 ms	0.997
time_to_load	0.85 ± 0.0092 s	0.843 ± 0.0079 s	1.01
utils/combine_operators	0.0608 ± 0.0036 ms	0.0604 ± 0.0037 ms	1.01
utils/convert/break_sharing	0.05 ± 0.0054 ms	0.051 ± 0.0066 ms	0.979
utils/convert/preserve_sharing	0.235 ± 0.0092 ms	0.236 ± 0.01 ms	0.998
utils/copy/break_sharing	0.0487 ± 0.0041 ms	0.0491 ± 0.0045 ms	0.992
utils/copy/preserve_sharing	0.234 ± 0.008 ms	0.235 ± 0.0084 ms	0.994
utils/count_constants	26.1 ± 0.7 μs	26.2 ± 0.7 μs	0.996
utils/count_depth	29.6 ± 0.8 μs	28.4 ± 0.8 μs	1.04
utils/count_nodes	25.1 ± 0.7 μs	25.5 ± 0.7 μs	0.984
utils/get_set_constants!	0.103 ± 0.0023 ms	0.104 ± 0.0023 ms	0.999
utils/has_constants	7.43 ± 0.54 μs	7.15 ± 0.52 μs	1.04
utils/has_operators	2.43 ± 0.078 μs	2.16 ± 0.07 μs	1.13
utils/index_constants	0.0805 ± 0.0041 ms	0.0814 ± 0.0044 ms	0.989
utils/is_constant	6.99 ± 0.41 μs	7.63 ± 0.43 μs	0.915
utils/simplify_tree	0.172 ± 0.016 ms	0.179 ± 0.017 ms	0.96
utils/string_tree	0.366 ± 0.012 ms	0.368 ± 0.011 ms	0.994

Benchmark Plots

A plot of the benchmark results have been uploaded as an artifact to the workflow run for this PR. Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).