Added libstd::io::fs::access()

[ghost]

As I was missing the posix access() function I added it to libstd::io::fs taking the existing functions there as an example.
I added the corresponding wrapper functions to the windows and unix libraries and tested it on windows x86 and linux x86_64.
Rebased to master and ran make check with no errors.

[alexcrichton]

Right now we're in the process of stabilizing the std::io::fs module, and I think that we may want to hold off on this for a few reasons at the moment:

• The permissions model of std::io::fs will likely change to a more platform-agnostic model than permission bits as it's not really what Windows does (only unix).
• In general we do not want to be calling MSVCRT functions but instead calling native Windows API functions.
• We're moving away from Unix-centric names to more descriptive and Windows-friendly names, so this may not want to be called just access.

Functionality such as this is likely to be provided unconditionally as part of the std::os::unix module at some point (raw platform-specific APIs), but it will likely not live in std::io::fs as-is for now. While we're hammering out these issues with the API, I'm going to close this for now, but I'll be sure to cc you when we open the std::io RFC so you can comment! Thanks for the PR regardless!