Add an "alloc" feature and make tests work with any feature combination

[Ogeon]

Split out a separate "alloc" feature, that can be used without "std". This allows allocations on platforms that still allow them without the std library.

Also makes it possible to run tests with any feature combination (without doc-tests, at the moment).

Closed Issues

• Fixes #366

[codecov[bot]]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (4c97fb5) 81.46% compared to head (6d96f73) 81.50%. Report is 2 commits behind head on master.

Files	Patch %	Lines
palette/src/okhsl.rs	96.07%	4 Missing :warning:
palette/src/okhwb.rs	96.96%	2 Missing :warning:
palette/src/hues.rs	99.05%	1 Missing :warning:
palette/src/okhsv.rs	99.41%	1 Missing :warning:
palette/src/oklch.rs	98.33%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #373 +/- ## ========================================== + Coverage 81.46% 81.50% +0.03% ========================================== Files 113 113 Lines 18438 18526 +88 Branches 18438 18526 +88 ========================================== + Hits 15021 15100 +79 - Misses 3220 3229 +9 Partials 197 197 ```

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[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #373 will degrade performances by 20.49%

_{:warning: No base runs were found}

_{Falling back to comparing alloc_and_any_features_in_tests (6d96f73) with master (4c97fb5)}

Summary

⚡ 1 improvements ❌ 1 regressions ✅ 43 untouched benchmarks

:warning: _Please fix the performance issues or acknowledge them on CodSpeed._

Benchmarks breakdown

	Benchmark	master	alloc_and_any_features_in_tests	Change
❌	matrix_inverse	215.6 ns	271.1 ns	-20.49%
⚡	multiply_3x3	427.8 ns	372.2 ns	+14.93%

[Ogeon]

@jonassmedegaard, you may want to have a look at this. I think I managed to find a good middle ground where the unit tests can run with any feature set. I didn't manage to get the doc-tests to work equally well, so I hope that's no issue.

A couple of things to keep in mind:

• ~The math traits will always use the std library during tests. Otherwise you would have to use either the "std" or "libm" feature. It may be possible to tweak it further to select libm if the "std" feature isn't set, though.~ Never mind, it should pick libm now, if it's requested without std.
• There are a couple of tests that require both "approx" and "alloc". I set up the tests here with cargo hack to also test pairs of features.

I don't know if you are able to test run your setup on this branch, but I hope it will let you skip the patch.