lib/fs: Optimize WindowsInvalidFilename (#8687)
lib/fs: Optimize WindowsInvalidFilename (#8687)

Replaced strings.Split with the new strings.Cut, which doesn't allocate.

name                           old time/op    new time/op    delta
WindowsInvalidFilenameValid-8     154ns ± 2%      89ns ± 0%   -42.09%  (p=0.000 n=10+9)
WindowsInvalidFilenameNUL-8       124ns ± 2%     124ns ± 1%      ~     (p=0.371 n=8+10)

name                           old alloc/op   new alloc/op   delta
WindowsInvalidFilenameValid-8     16.0B ± 0%      0.0B       -100.00%  (p=0.000 n=10+10)
WindowsInvalidFilenameNUL-8       19.0B ± 0%      3.0B ± 0%   -84.21%  (p=0.000 n=10+10)

name                           old allocs/op  new allocs/op  delta
WindowsInvalidFilenameValid-8      1.00 ± 0%      0.00       -100.00%  (p=0.000 n=10+10)
WindowsInvalidFilenameNUL-8        2.00 ± 0%      1.00 ± 0%   -50.00%  (p=0.000 n=10+10)
syncthing/syncthing,syncthing/syncthing,syncthing/syncthing,syncthing/syncthing,syncthing/syncthing,syncthing/syncthing