Add support for direct conversion between certain bases

[Skalman]

E.g. converting from binary to hexadecimal shouldn't go via the internal number presentation.

Suggested fast conversions:

• 2 <-> 16
• 10 <-> 100

And possibly a few more, especially if they can be created dynamically:

• 2 <-> 4 <-> 8 <-> 16 <-> 64
• 3 <-> 9
• 5 <-> 25
• 6 <-> 36
• 10 <-> 100 <-> 1000

[Skalman]

Support for directly converting between bases was added in da8793a, but these conversions weren't added.