Custom input mapping

[cbrgray]

Debug menu feature, probably requiring a new 'page' like the warps menu. Allows remapping of button combos e.g. create/load state.
For persistence, they should be stored in SRAM.

Since SRAM can become corrupted, the input to open the debug menu should not be mappable. Additionally, a backup of the mappings should also be stored in SRAM to enable detection if corruption occurs, via comparison on boot. If corruption is detected, clear any custom mappings back to default (and maybe set a flag to alert the user?).

[cbrgray]

Implemented in f67fe72 and d3f4ec0