Windows improvements and various other updates

This releases fixes several issues with the package on Windows:

• #29 - Avoid leaking file handles when MapViewOfFile fails.
• #22 - Fix anonymous mappings on Windows (bug introduced by the fix to #5).
• #5 - Fix Flush on Windows.

Other updates:

• Update golang.org/x/sys for compatibility with the upcoming Go 1.18 release.
• Enable compilation for Plan 9. (#25) Note that Plan 9 still does not work at runtime.
• Add examples.
• Add GitHub Actions CI.

Stable release

I thought it was finally time to make a proper release of this package since it's had a stable interface for ages and is modestly popular.

I did a few things before cutting this release:

• Switched from syscall to golang.org/x/sys, which automatically adds support for a variety of Unixes
• Merged a fix for a longstanding Windows bug