Create physical constants submodule

[MilesCranmer]

PR for #26.

1. This creates a Constants submodule.
2. Units and Constants are now both exported by DynamicQuantities (unsure about this, but feels right)
3. Constants is made available within the @u_str macro for ease of access. (Some constants are treated like units)

For example:

julia> mass_energy_of_sun = 1.0 * u"Const.M_sun * Const.c^2"
1.7870936681139022e47 m² kg s⁻²

julia> q1 = 1.5u"C"; q2 = 1e-5u"C";

julia> F = Const.k_e * q1 * q2 / (1u"m")
134813.2768839176 m² kg s⁻²

julia> f = 500u"MHz"
5.0e8 s⁻¹

julia> (Constants.h * f) / Constants.eV
2.0678338484619293e-6

The following constants are included.

• c: Speed of light in a vacuum. Standard.
• h: Planck constant. Standard.
• hbar: Reduced Planck constant (h/2π). Standard.
• e: Elementary charge. Standard.
• k_B: Boltzmann constant. Standard.
• N_A: Avogadro constant. Standard.
• eV: Electron volt. Standard.
• R: Molar gas constant. Standard.
• F: Faraday constant. Standard.
• sigma_sb: Stefan-Boltzmann constant. Standard.
• alpha: Fine-structure constant. Measured.
• u: Atomic mass unit (1/12th the mass of Carbon-12). Measured.
• G: Newtonian constant of gravitation. Measured.
• mu_0: Vacuum magnetic permeability. Measured.
• eps_0: Vacuum electric permittivity. Measured.
• m_e: Electron mass. Measured.
• m_p: Proton mass. Measured.
• m_n: Neutron mass. Measured.
• a_0: Bohr radius. Measured.
• k_e: Coulomb constant (Note: SI units only!). Measured.
• Ryd: Rydberg frequency. Measured.

I included the most common astro ones too (It might be nice if someone added similar sets of very common constants from other fields.)

• M_earth: Earth mass. Measured.
• M_sun: Solar mass. Measured.
• M_jup: Jupiter mass. Measured.
• R_earth: Nominal Earth equatorial radius. Standard.
• R_jup: Nominal Jupiter equatorial radius. Standard.
• R_sun: Nominal solar radius. Standard.
• L_sun: Nominal solar luminosity. Standard.
• L_bol0: Standard luminosity at absolute bolometric magnitude 0. Standard.
• sigma_T: Thomson scattering cross-section. Measured.
• au: Astronomical unit. Standard.
• pc: Parsec. Standard.
• ly: Light year. Standard.
• atm: Standard atmosphere. Standard.

"Standard" means it is an exact standard, while "Measured" means there is some uncertainty associated with it.

cc @odow @ChrisRackauckas. I am eager to hear your take on this interface given your interest in using the package downstream.

[github-actions]

Benchmark Results

	main	a12d87a...	t[main]/t[a12d87a...]
creation/Quantity(x)	3 ± 0.1 ns	3.1 ± 0.1 ns	0.968
creation/Quantity(x, length=y)	3.7 ± 0.001 ns	3.4 ± 0 ns	1.09
time_to_load	0.126 ± 0.0013 s	0.131 ± 0.00045 s	0.967
with_numbers/*real	3.4 ± 0 ns	3.4 ± 0 ns	1
with_numbers/^int	24.1 ± 2.4 ns	24.1 ± 2.4 ns	1
with_numbers/^int * real	24.1 ± 2.4 ns	24.1 ± 2.4 ns	1
with_quantity/+y	6.8 ± 0.1 ns	7.7 ± 0.001 ns	0.883
with_quantity//y	4.3 ± 0.1 ns	3.4 ± 0 ns	1.26
with_self/dimension	1.7 ± 0 ns	1.7 ± 0 ns	1
with_self/inv	3.4 ± 0 ns	3.4 ± 0 ns	1
with_self/ustrip	1.7 ± 0 ns	1.7 ± 0 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).

[odow]

From JuMP's point of view, I think most people would just be using the distance, mass, and time units. No fancy physics stuff.

[MilesCranmer]

Combining with verbatim units branch, so closing.

[MilesCranmer]

Continued on #32. This PR also introduces the symbolic dimensions to help with trulsf/UnitJuMP.jl#18