Make default parametric denominator of FixedRational `::Int64`

[MilesCranmer]

This fixes a weird issue where the user would write out a quantity explicitly, like:

user_quantity = Quantity(10.0, Dimensions{FixedRational{Int32,25200}}(1, 0, 0, 0, 0, 0, 0))

and be surprised when this is not the same type as 10u"m". The reason for this is that FixedRational{Int32,25200}} != FixedRational{Int32,Int32(25200)}}!

So, this sets the default denominator to have Int64 type so that the user can write out an integer and not be surprised when it's a different type from the standard.

[github-actions[bot]]

Benchmark Results

	main	37be60fd7daf91...	main/37be60fd7daf91...
Quantity/creation/Quantity(x)	4.32 ± 0.92 ns	3.71 ± 0.92 ns	1.16
Quantity/creation/Quantity(x, length=y)	3.11 ± 0.01 ns	3.41 ± 0.01 ns	0.912
Quantity/with_numbers/*real	3.11 ± 0.01 ns	3.1 ± 0.01 ns	1
Quantity/with_numbers/^int	8.98 ± 2.2 ns	8.37 ± 2.2 ns	1.07
Quantity/with_numbers/^int * real	9.6 ± 2.2 ns	8.05 ± 1.9 ns	1.19
Quantity/with_quantity/+y	4.04 ± 0.001 ns	4.04 ± 0.001 ns	1
Quantity/with_quantity//y	3.42 ± 0.01 ns	3.11 ± 0.001 ns	1.1
Quantity/with_self/dimension	3.1 ± 0.01 ns	2.79 ± 0.009 ns	1.11
Quantity/with_self/inv	3.11 ± 0.001 ns	3.11 ± 0.001 ns	1
Quantity/with_self/ustrip	2.79 ± 0.01 ns	2.79 ± 0.01 ns	1
QuantityArray/broadcasting/multi_array_of_quantities	0.145 ± 0.0006 ms	0.15 ± 0.00092 ms	0.969
QuantityArray/broadcasting/multi_normal_array	0.0622 ± 0.00028 ms	0.0499 ± 0.00018 ms	1.25
QuantityArray/broadcasting/multi_quantity_array	0.155 ± 0.00084 ms	0.155 ± 0.00059 ms	1
QuantityArray/broadcasting/x^2_array_of_quantities	25.8 ± 2.8 μs	24.7 ± 2.3 μs	1.05
QuantityArray/broadcasting/x^2_normal_array	6.02 ± 0.54 μs	5.84 ± 0.62 μs	1.03
QuantityArray/broadcasting/x^2_quantity_array	6.96 ± 0.25 μs	7.04 ± 0.29 μs	0.989
QuantityArray/broadcasting/x^4_array_of_quantities	0.0788 ± 0.00071 ms	0.0788 ± 0.00059 ms	1
QuantityArray/broadcasting/x^4_normal_array	0.0561 ± 0.00021 ms	0.0499 ± 0.0002 ms	1.12
QuantityArray/broadcasting/x^4_quantity_array	0.0594 ± 0.003 ms	0.05 ± 0.0002 ms	1.19
time_to_load	0.134 ± 0.0029 s	0.128 ± 0.0028 s	1.05

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).