Give `Dimensions` a type parameter.

[MilesCranmer]

Okay I think I got it working. Could you add some unittests for custom Dimensions{R} types? Maybe the Int8/6 idea that you mentioned?

[MilesCranmer]

Not sure why coveralls is failing but otherwise the tests are passing.

[MilesCranmer]

Should probably test when and where type promotion happens, and if it gives expected behavior or not.

[github-actions[bot]]

Benchmark Results

	main	5d79a34ab77734...	t[main]/t[5d79a34ab77734...]
creation/Quantity(x)	5.6 ± 0.5 ns	4.3 ± 0.2 ns	1.3
creation/Quantity(x, length=y)	5.1 ± 0.2 ns	4.6 ± 0.2 ns	1.11
time_to_load	0.176 ± 0.0069 s	0.0922 ± 0.01 s	1.91
with_numbers/*real	4.5 ± 0.3 ns	4.3 ± 0.1 ns	1.05
with_numbers/^int	0.0335 ± 0.0036 μs	0.247 ± 0.026 μs	0.135
with_numbers/^int * real	0.0344 ± 0.0039 μs	0.248 ± 0.024 μs	0.139
with_quantity/+y	22.7 ± 1.7 ns	0.333 ± 0.034 μs	0.0682
with_quantity//y	23 ± 1.7 ns	0.34 ± 0.039 μs	0.0677
with_self/dimension	1.8 ± 0.1 ns	2 ± 0.1 ns	0.9
with_self/inv	20.8 ± 1.6 ns	9.7 ± 0.3 ns	2.14
with_self/ustrip	1.9 ± 0.1 ns	2 ± 0.1 ns	0.95

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]

Okay I think it's ready. Could you review my changes @oscardssmith ?

[oscardssmith]

LGTM