v2.3.1: Windows platform update; minimum rust version specified

This release includes a contribution by @russellbanks which changes windows to use the windows-sys crate instead of winapi, as suggested by @thewh1teagle.

Specifies a minimum stable rust version of 1.68, which is the one that was current when this major version was released. Only the library is warranted to build on 1.68. In particular, none of the tests or examples will work; they require 1.70 for dev dependencies.