Allow the `bit`/`res`/`set` bit index to be determined at link time

[Rangi42]

Based on ISSOtm/rsgbds#8

I expect this would require a new RPN operator or three for bit/res/set opcodes, checking their value to be in 0-7 range the way operators like RST and HRAM check for appropriate ranges. That would need to increment the object file revision number to 12, but that should be okay, we've done so before (0.4.2 incremented to r6, 0.5.1 incremented to r8).