Included dispatch to toexpr that creates Tuples correctly

[raphaelchinchilla]

This dispatch allows Symbolics.build_function to build functions when the targets are Tuples. It includes a test.

This closes the issue 46 in Symbolics. I also made some comments there about it.

[github-actions]

Benchmark Results

	master	564ba54...	t[master]/t[564ba54...]
overhead/acrule/a+2	1.01 ± 0.31 μs	1.06 ± 0.29 μs	0.949
overhead/acrule/a+2+b	1.01 ± 0.27 μs	1.03 ± 0.28 μs	0.985
overhead/acrule/a+b	0.368 ± 0.092 μs	0.383 ± 0.093 μs	0.96
overhead/acrule/noop:Int	23.5 ± 2.9 ns	24.8 ± 2.9 ns	0.945
overhead/acrule/noop:Sym	0.0473 ± 0.0081 μs	0.0478 ± 0.011 μs	0.989
overhead/rule/noop:Int	0.0396 ± 0.014 μs	0.039 ± 0.017 μs	1.02
overhead/rule/noop:Sym	0.0748 ± 0.021 μs	0.0802 ± 0.021 μs	0.933
overhead/rule/noop:Term	0.0665 ± 0.012 μs	0.0769 ± 0.021 μs	0.865
overhead/ruleset/noop:Int	0.153 ± 0.039 μs	0.147 ± 0.028 μs	1.04
overhead/ruleset/noop:Sym	0.193 ± 0.043 μs	0.196 ± 0.045 μs	0.986
overhead/ruleset/noop:Term	5.66 ± 0.99 μs	5.3 ± 1.2 μs	1.07
overhead/simplify/noop:Int	0.218 ± 0.056 μs	0.245 ± 0.06 μs	0.89
overhead/simplify/noop:Sym	0.248 ± 0.053 μs	0.244 ± 0.051 μs	1.02
overhead/simplify/noop:Term	0.077 ± 0.02 ms	0.0739 ± 0.018 ms	1.04
overhead/simplify/randterm (+, *):serial	0.175 ± 0.0083 s	0.17 ± 0.012 s	1.03
overhead/simplify/randterm (+, *):thread	0.113 ± 0.014 s	0.107 ± 0.016 s	1.06
overhead/simplify/randterm (/, *):serial	0.452 ± 0.094 ms	0.409 ± 0.075 ms	1.11
overhead/simplify/randterm (/, *):thread	0.54 ± 0.099 ms	0.482 ± 0.08 ms	1.12
overhead/substitute/a	0.112 ± 0.03 ms	0.102 ± 0.022 ms	1.09
overhead/substitute/a,b	0.106 ± 0.023 ms	0.0963 ± 0.021 ms	1.1
overhead/substitute/a,b,c	27.9 ± 5.7 μs	26.8 ± 3.6 μs	1.04
polyform/easy_iszero	0.0657 ± 0.018 ms	0.0641 ± 0.017 ms	1.02
polyform/isone	2.6 ± 0.2 ns	2.6 ± 0.2 ns	1
polyform/iszero	2.33 ± 0.36 ms	2.34 ± 0.37 ms	0.996
polyform/simplify_fractions	3.16 ± 0.49 ms	3.14 ± 0.49 ms	1.01
time_to_load	5.58 ± 0.11 s	5.55 ± 0.15 s	1.01

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).