Improve case-sensitive path comparison

[ForNeVeR]

I suggest the following changes.

1. Introduce three different path comparator kinds.

   • [x] Textual only. This one should operate on strict string equality, and named accordingly (something like StrictStringPathComparer?).

   • [x] Platform-default comparer: should implement case-sensitive comparison on Linux, and case-insensitive (probably with corresponding relaxations related to Unicode normalization) on Windows and macOS.

   • [ ] File-system-aware comparer: for each compared path component, should compare the actual case sensitivity of the corresponding file system subroot. For non-existent paths, it should use the platform-dependent policy of calculating the case sensitivity for new subdirectories (is it normally taken from the parent directory?).

     This one is obviously IO-intensive, so I'm thinking of introducing some sort of "sensitivity cache" that'd store the lists of checked paths and subtrees in a trie data structure, and would be used for one or multiple operations (probably one per comparer instance, with the ability of manual reset).

2. Allow the paths to use different comparers; platform-default being used by default, as one giving the best precision while not losing performance dur to intensive IO.