HP-41: OS4 module and Ladybug 1A

[hth]

This is to announce the first release of the OS4 module. It is a banked module that must be loaded to page 4 as it uses the 0x4000 vector.

OS4 aims to provide a lot of new features that can transform the HP-41 quite dramatically, while at the same time preserving all the behavior we like to keep as is.

At a very high level OS4 does the following:

1. Removes the 64 function limitation for a single plug-in module page.
2. Support for programmable prompting XROM functions with one or two postfix arguments.
3. Provides a robust way of defining new alternative keyboard layouts, supporting both full and partial layouts without relying on key assignments.
4. A way to override the default display of the X register with something else.
5. Support for writing temporary modes, such as input modes, updating clock style modes and catalogs. The interval timer can be borrowed (if timer chip present) to allow for periodic updates and timeouts in such modes.
   

On its own OS4 will not do much as it is in page 4. You will need other modules to enable its features. Two such modules are available now, Ladybug 1A and Boost 0A (early snapshot release). Ladybug provides a integer mode for the HP-41, much like the HP-16C. Boost has XEQ and ASN replacements that allow for direct execution and assignment of secondary functions, plus some other goodies. Boost also provides an extensible catalog mechanism and adds one new catalog (so far), number 07 which shows buffers.

You can also use OS4 for developing your own modules.

References:
https://github.com/hth313/OS4/releases
https://github.com/hth313/ladybug/releases
https://github.com/hth313/boost41/releases