Add conversion matrix and Convert{Once} traits

[Ogeon]

Adds a few basic tools for reusable converters, similar to what's discussed in #396:

• convert::Matrix3, which represents a 3x3 conversion matrix from one type to another. It may speed up multi-step conversions that can be simplified as a single matrix.
• convert::Convert and convert::ConvertOnce traits for abstracting over types that can convert other types, similar to Fn and FnOnce in relation to each other. They are implemented by convert::Matrix3 and cam16::BakedParameters.
• Rgb::matrix_from_xyz, Xyz::matrix_from_rgb, Xyz::matrix_from_lms and Lms::matrix_from_xyz as initial building blocks for composing conversion matrices.

Some old matrix code has also been cleaned up or replaced. The matrix module is not considered public, so its content should not be relied upon.

[codspeed-hq]

CodSpeed Performance Report

Merging #406 will degrade performances by 11.73%

_{Comparing conversion_matrix (b907852) with master (7971660)}

Summary

❌ 1 regressions
✅ 42 untouched benchmarks

🆕 4 new benchmarks
⁉️ 3 (👁 3) dropped benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	master	conversion_matrix	Change
🆕	linsrgb to xyz - Matrix3	N/A	5.9 µs	N/A
❌	matrix_inverse	438.9 ns	497.2 ns	-11.73%
👁	multiply_rgb_to_xyz	0 s	N/A	N/A
👁	multiply_xyz	0 s	N/A	N/A
👁	multiply_xyz_to_rgb	0 s	N/A	N/A
🆕	lms to linsrgb	N/A	3.2 µs	N/A
🆕	lms to linsrgb - Matrix3	N/A	2.2 µs	N/A
🆕	xyz to linsrgb - Matrix3	N/A	5.9 µs	N/A

[codecov]

Codecov Report

Attention: Patch coverage is 87.87879% with 36 lines in your changes missing coverage. Please review.

Project coverage is 82.62%. Comparing base (7971660) to head (b907852).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #406      +/-   ##
==========================================
- Coverage   82.66%   82.62%   -0.04%     
==========================================
  Files         127      128       +1     
  Lines       19539    19688     +149     
  Branches    19539    19688     +149     
==========================================
+ Hits        16151    16267     +116     
- Misses       3285     3315      +30     
- Partials      103      106       +3     

Components	Coverage Δ
palette	82.67% <87.87%> (-0.04%)	⬇️
palette_derive	81.98% <ø> (ø)