Create (1) physical constants, and (2) symbolic dimensions object

[MilesCranmer]

This PR is the continuation of #27, fixing #26.

It also now includes a symbolic dimensions object, to help with the JuMP.jl requirements: trulsf/UnitJuMP.jl#18. This means you can operate with units without needing to convert to base units, like:

julia> us"km/h"
1.0 km h⁻¹

julia> expand_units(us"km/h")  # equivalent of u"km/h"
0.2777777777777778 m s⁻¹

TODO:

• Add back docs on units
• Verify constants docstrings show up on documentation
• Add docs page on symbolic dimensions
• Enable constants with namespace collisions, equal to their expanded forms

[github-actions]

Benchmark Results

	main	e6bd940...	t[main]/t[e6bd940...]
creation/Quantity(x)	3.2 ± 0.1 ns	2.9 ± 0.1 ns	1.1
creation/Quantity(x, length=y)	3.3 ± 0.1 ns	3.3 ± 0.1 ns	1
time_to_load	0.123 ± 0.0031 s	0.133 ± 0.00024 s	0.929
with_numbers/*real	2.9 ± 0.1 ns	3.3 ± 0.1 ns	0.879
with_numbers/^int	20.9 ± 1.6 ns	20.7 ± 1.4 ns	1.01
with_numbers/^int * real	21.2 ± 1.4 ns	21.3 ± 1.7 ns	0.995
with_quantity/+y	5.7 ± 0.2 ns	5.7 ± 0.2 ns	1
with_quantity//y	3.7 ± 0.1 ns	3.7 ± 0.1 ns	1
with_self/dimension	1.6 ± 0.1 ns	1.6 ± 0.1 ns	1
with_self/inv	3.3 ± 0.1 ns	3.7 ± 0.1 ns	0.892
with_self/ustrip	1.6 ± 0.1 ns	1.6 ± 0.1 ns	1

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).

[MilesCranmer]

After merging, can add some stuff to the README about this.