(CFACT-121) Implement dynamic loading of the ruby dll on Windows

Adds defines for Windows _access and _waccess calls, to make them similar to Linux.
Adds a scoped_error class for Windows system error messages.
Adds execution::which, implemented identically to Facter's method.
Simplifies POSIX api::find_library with use of tie for comparison and assignment.
Implements Windows api::find_library, dynamic_library::load, and dynamic_library::find_symbol to facilitate loading the Ruby dll.
True dat. Also I was a bit confused, but rubyinstaller.org doesn't even produce a 1.9.3 x64 so that's kinda moot. (I was under the impression previously that there was 1.9.3 x64 dll which was buggy. But it doesn't even exist AFAICT.)