I split the "small" source into its own file. I think it could
become smaller, but not without changes that make #ifdefs
untenable.
The "small" version need a couple of fixes (define "savsrc",
branch to "advsrc1" not "advsrc").
Merlin apparently detects SEC+XCE and automatically switches back
to 8-bit registers, so I added an MX %00 to the 65816 version
after the exit section.
Added comments.
This just updates the git copy of the sources. An updated set of
disk images will appear soonish.