DB48X: HP48-like RPL implementation for DM42

[c3d]

New release 0.4 is out. https://github.com/c3d/DB48X-on-DM42/releases/tag/v0.4
Many changes in this release. The main ones are:

1. Support for matrix operations (arithmetic, det, norm, inverse)
2. Support for equation rewrite, with early implementation of collect and expand.
3. Transient alpha mode: hold up or down arrow with a letter key to quickly type uppercase or lowercase

Please give it a whirl. I will post a video about it soon.