Complete Windows port

[dwarring]

LibXML release adds 0.6.11 initial support for Windows. DLLs are installed for libxml and xml6, so there's no need to build anything.

Some loose ends:

1. Rebuilding of xml6.dll is currently a manual process. Needs an automated workflow, similar to, but more involved than, Base64::Native.
2. libxml2.dll has been built with iconv disabled, which limits XML encoding to a few common cases such as utf-8 and latin-1.
3. Also built with no threading support

Also awaiting PR's on LibaryMake to fix install failures. Currently needs to be zef installed either --force or -/test

[dwarring]

Note also the approach taken by Gtk::Simple, which downloads DLLs from http://gtk-dlls.p6c.org, including libxml2-2.dll

[dwarring]

Currently tests run, but fails on installation, see this test which has LibXML as a dependency.

Caused by name mangling. xml6 is searching for libxml.dll, but it's been renamed during installation.

Again, Gtk::Simple may offer a solution, see GTK::Simple::NativeLib.rakumod which takes copies of the DLLs to a temporary directory with unmangled names.