This also switches them to being 64-bit, which I think is
probably acceptable in this modern age, especially for such
a non-essential piece of software. If anyone complains I can
always reinstate a parallel 32-bit build.
To support the switch to 64-bit, this commit also changes the default
install directory in the MSI to 'Program Files' rather than 'Program
Files (x86)'.
I've reused most of the install script I wrote for PuTTY recently,
minus the selectable-features dialog, and plus some preliminary Mason
templating to automatically build the right set of puzzle binaries
into the installer.
Stable GUIDs are autogenerated by the same technique I use in PuTTY's
Visual Studio project file generation: hash a fixed pile of randomly
generated bits (that is, randomly generated _once_ and used forever)
with each filename or other identifier and use those as your random
number source.
