[jeremydubreil]

On Windows, infer cannot take paths apart, which makes it loop forever and run out of memory as soon as it normalizes a path such as C:/infer/...:

• Filename.parts never terminates on a path with a drive letter, as Filename.dirname "C:/" is "C:/" again, nor on a path rooted at a backslash, as Filename.dirname "\\" is "\\" again.
• Windows system calls such as getcwd answer with \ as separator, and Unix.realpath even answers in the extended-length form \?\C:\dir\file (\?\UNC\server\share\file for a network path). infer builds its paths with ^/ and compares them as strings, so it needs a single form.

This pull request adds four helpers to Utils:

• of_win32_path brings a Windows path back to the C:/dir/file form;
• split_drive splits off the drive letter on Windows, so that Filename.parts only sees the rest of the path;
• path_parts and of_path_parts split and join paths with the drive letter kept in front of the root part, and always join with /, as Filename.dir_sep is a backslash on Windows.

normalize_path_from, flatten_path and filename_to_relative use them, and realpath returns paths in the C:/dir/file form on Windows. In flatten_path, the drive letter of C:/ is kept without the :, which a file name cannot contain.

On Linux and macOS, split_drive returns the path unchanged, so these functions give the same results as before.