dir_entry: Implement lscolors::Colorable

[tavianator]

Now we can finally avoid calling stat() unless actually necessary to color a path. For example, with LS_COLORS="ex=0:sg=0:su=0:st=0:ow=0:tw=0:"

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
./fd-master . ~/code/linux -u -j1 --color=always	225.8 ± 6.4	209.1	231.6	1.60 ± 0.10
./fd-colorable . ~/code/linux -u -j1 --color=always	140.9 ± 8.3	122.6	150.0	1.00

And still a (slight) improvement with my default LS_COLORS:

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
./fd-master . ~/code/linux -u -j1 --color=always	259.2 ± 5.6	243.5	265.4	1.02 ± 0.04
./fd-colorable . ~/code/linux -u -j1 --color=always	255.2 ± 7.5	238.0	262.4	1.00

[sharkdp]

Cool! Thank you for this (and the related work in lscolors)!

[sharkdp]

I think this would be worth a ChangeLog entry.

[tavianator]

Doing colors in parallel is a gigantic win:

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
./fd-master . ~/code/linux --color=always	272.8 ± 14.6	244.2	287.5	2.31 ± 0.15
./fd-colorable . ~/code/linux --color=always	118.0 ± 4.3	108.8	125.5	1.00

No significant difference without colors:

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
./fd-master . ~/code/linux	144.5 ± 4.4	136.7	151.5	1.00
./fd-colorable . ~/code/linux	145.3 ± 3.7	138.2	150.9	1.01 ± 0.04

[sharkdp]

Doing colors in parallel is a gigantic win:

Oh wow, very nice!