Large set of changes to how PATH is handled. Changed fish to no longer modify PATH in share/config.fish. Introduced variable fish_user_paths, and a glue function __fish_reconstruct_path that splices together PATH with fish_user_paths. Changed fish to no longer validate changes to PATH unless the paths are new (i.e. don't recheck what's already there). Modified certain sets to store const wchar_t instead of wcstring to save a few allocations.
https://github.com/fish-shell/fish-shell/issues/527
Add a note to the documentation about fish_user_paths