Split Shade into Lighten and Darken, and add *Assign variants

[Ogeon]

I have split up Shade and given the traits more obvious names. I think the whole setup is nicer like this. Also added *Assign traits for them with implementations for [T].

Breaking Change

The Shade trait has been split up into Lighten and Darken, and Scalar is no longer implied to implement Float.

[github-actions]

Benchmark for 578507b

Click to view benchmark

Test	PR Benchmark	Master Benchmark	%
Cie family/lab to lch	3.5±0.05µs	3.8±0.15µs	-7.89%
Cie family/lab to xyz	950.6±19.37ns	919.5±24.09ns	+3.38%
Cie family/lch to lab	2.5±0.03µs	2.6±0.32µs	-3.85%
Cie family/linsrgb to xyz	3.8±0.10µs	3.7±0.12µs	+2.70%
Cie family/xyz to lab	11.1±0.42µs	11.8±0.50µs	-5.93%
Cie family/xyz to yxy	705.8±16.68ns	708.6±21.95ns	-0.40%
Cie family/yxy to xyz	633.0±31.84ns	602.2±18.79ns	+5.11%
Matrix functions/matrix_inverse	13.7±0.51ns	13.1±0.34ns	+4.58%
Matrix functions/multiply_3x3	10.7±0.17ns	18.5±0.52ns	-42.16%
Matrix functions/multiply_rgb_to_xyz	4.7±0.10ns	4.8±0.13ns	-2.08%
Matrix functions/multiply_xyz	4.7±0.12ns	4.7±0.12ns	0.00%
Matrix functions/multiply_xyz_to_rgb	4.7±0.11ns	4.7±0.11ns	0.00%
Matrix functions/rgb_to_xyz_matrix	24.8±0.62ns	24.7±0.71ns	+0.40%
Rgb family/hsl to hsv	766.9±18.38ns	761.8±19.73ns	+0.67%
Rgb family/hsl to linear hsl	9.4±0.20µs	9.4±0.20µs	0.00%
Rgb family/hsl to rgb	2.5±0.05µs	2.5±0.06µs	0.00%
Rgb family/hsv to hsl	1126.3±23.68ns	1151.9±39.46ns	-2.22%
Rgb family/hsv to hwb	265.4±6.21ns	265.9±6.36ns	-0.19%
Rgb family/hsv to linear hsv	9.0±0.44µs	8.7±0.25µs	+3.45%
Rgb family/hsv to rgb	2.4±0.06µs	2.4±0.06µs	0.00%
Rgb family/hwb to hsv	558.2±13.48ns	553.9±9.61ns	+0.78%
Rgb family/hwb to linear hwb	10.8±0.21µs	10.9±0.25µs	-0.92%
Rgb family/linear hsl to hsl	10.6±0.47µs	10.0±0.23µs	+6.00%
Rgb family/linear hsv to hsv	9.6±0.41µs	9.7±0.18µs	-1.03%
Rgb family/linear hwb to hwb	11.9±0.53µs	12.0±0.52µs	-0.83%
Rgb family/linsrgb to rgb	4.6±0.11µs	4.6±0.11µs	0.00%
Rgb family/linsrgb_f32 to rgb_u8	6.4±0.49µs	6.2±0.14µs	+3.23%
Rgb family/rgb to hsl	851.4±24.26ns	843.7±20.79ns	+0.91%
Rgb family/rgb to hsv	644.8±18.37ns	656.9±15.84ns	-1.84%
Rgb family/rgb to linsrgb	5.0±0.13µs	5.0±0.21µs	0.00%
Rgb family/rgb_u8 to linsrgb_f32	5.5±0.26µs	5.3±0.12µs	+3.77%
Rgb family/xyz to linsrgb	8.9±0.29µs	8.9±0.19µs	0.00%

[Ogeon]

bors r+

[bors]

Build succeeded:

• ci