Cygwin recently did an update that broken string_span again. The original update had to provide a custom strnlen function because Cygwin doesn't implement this (it's a non-std extension). We never added wchar_t support, and the Cygwin update now breaks on exactly this. This patch provides the missing wchar_t.
Not all platforms have support for strnlen (e.g. cygwin) as this is a non-standard function. This patch provides a custom implementation of strnlen for these platforms.
