Clean up type utilities: `constructorof`, `with_type_parameters`, `dimension_names`

[MilesCranmer]

This improves the type utilities, making it easier for a user to set up custom behavior in an abstract type:

• constructorof for creating new types
• with_type_parameters for changing the value or dimension type in a quantity type
• dimension_names for getting the names of all the dimensions instead of using propertynames

---

TODO:

• [x] Document constructorof and with_type_parameters
• [x] Add example to docs on defining custom versions of these

---

Edit 1: I'm going to say this will depend on https://github.com/JuliaObjects/ConstructionBase.jl/pull/81 so that we don't make LinearAlgebra.jl a hard dependency.

Edit 2: I'll just keep it as a custom constructorof to avoid having extra dependencies

[MilesCranmer]

I'd like to get this into v0.8 since it's a breaking change. So let me know soon if there are any other comments...

[github-actions[bot]]

Benchmark Results

	main	aec9a125f96008...	t[main]/t[aec9a125f96008...]
Quantity/creation/Quantity(x)	3.2 ± 0.1 ns	3.2 ± 0.1 ns	1
Quantity/creation/Quantity(x, length=y)	3.3 ± 0.1 ns	3.3 ± 0.1 ns	1
Quantity/with_numbers/*real	2.9 ± 0.1 ns	2.9 ± 0.1 ns	1
Quantity/with_numbers/^int	10.1 ± 2.5 ns	10.1 ± 2.8 ns	1
Quantity/with_numbers/^int * real	10.5 ± 2.1 ns	10.5 ± 2.4 ns	1
Quantity/with_quantity/+y	5.9 ± 0.1 ns	5.9 ± 0.1 ns	1
Quantity/with_quantity//y	3.3 ± 0.1 ns	3.3 ± 0.1 ns	1
Quantity/with_self/dimension	1.6 ± 0.1 ns	1.6 ± 0.1 ns	1
Quantity/with_self/inv	3.3 ± 0.1 ns	3.3 ± 0.1 ns	1
Quantity/with_self/ustrip	1.6 ± 0.1 ns	1.6 ± 0.1 ns	1
QuantityArray/broadcasting/multi_array_of_quantities	0.198 ± 0.17 ms	0.197 ± 0.17 ms	1.01
QuantityArray/broadcasting/multi_normal_array	0.0682 ± 0.0017 ms	0.0681 ± 0.0008 ms	1
QuantityArray/broadcasting/multi_quantity_array	0.228 ± 0.0023 ms	0.228 ± 0.0026 ms	1
QuantityArray/broadcasting/x^2_array_of_quantities	0.0428 ± 0.0041 ms	0.0406 ± 0.0037 ms	1.05
QuantityArray/broadcasting/x^2_normal_array	8.4 ± 1.2 μs	8.1 ± 1 μs	1.04
QuantityArray/broadcasting/x^2_quantity_array	9.6 ± 1.3 μs	10.4 ± 0.9 μs	0.923
QuantityArray/broadcasting/x^4_array_of_quantities	0.117 ± 0.0044 ms	0.117 ± 0.0043 ms	0.994
QuantityArray/broadcasting/x^4_normal_array	0.0623 ± 0.0026 ms	0.062 ± 0.0006 ms	1
QuantityArray/broadcasting/x^4_quantity_array	0.0819 ± 0.0036 ms	0.0829 ± 0.0041 ms	0.988
time_to_load	0.202 ± 0.0011 s	0.201 ± 0.0021 s	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).