DB48X: HP48-like RPL implementation for DM42

[c3d]

Pre-release v0.2.3 is here: https://github.com/c3d/DB48X-on-DM42/rel...tag/v0.2.3.

Bug fixes:

- Memory corruption in division of large numbers (#70)
- Parsing 'X⁻¹+(Y²+Z³)' resulting in syntax error (#76)
- Parsing -80/60 resulting in 4/3 (sign lost) (#78)
- Large numbers written to state file had separators (#83)
- Render names on stack as 'ABC' instead of 'ABC' (#84)
- Evaluate names as 'ABC' and not ABC (#86)
- Directory corruption following purge (#87)
- Local blocks now always execute (#90)
- Correct spacing of control structures on stack

Features:

- Keyboard binding to create lists (#89)
- Implementation of for loops (#92)