Fix off-by-one and char encoding issue #3649 #3702

[mungre]

As described in #3649, stack fails on Windows when building project dependencies because the paths it generates for cached files are too long. #3702 and 36c1226 are fixes to this but contain an off-by-one error so builds still fail when a cache path is exactly 260 characters long. This patch fixes the off-by-one error.

Additionally, the Windows 260 character path limit is counted in 16-bit utf-16 code units, not unicode characters. This patch also addresses this.

This is only tested on Windows, but the non-Windows functionality should not have changed.

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

To test this I rebuilt stack and a simple yesod app and all their dependencies using this modified stack. This had previously failed. I also edited the CPP constant to ensure the non-Windows branch of the code compiles without error.

[mgsloan]

This looks great, @mungre, thanks! My bad on the delay in merging.