Add individual bit/flag constants for audio registers

[Rangi42]

Version 2.8 (0b46188) added bit/flag constants for the audio register aliases, but did so in an unusual way: since many audio registers are basically the same but for different channels, it reused constants.

• AUDLENF_DUTY applies to AUD1LEN (NR11) and AUD2LEN (NR21)
  • (...but not AUD3LEN (NR31) or AUD4LEN (NR41)!)
• AUDLENF_TIMER applies to AUD1LEN (NR11), AUD2LEN (NR21), and AUD4LEN (NR41)
  • (...but not AUD3LEN (NR31)!)
• AUDENV_* apply to AUD1ENV (NR12), AUD2ENV (NR22), and AUD4ENV (NR42)
  • (there is no AUD3ENV!)
• AUDHIGH_* apply to AUD1HIGH (NR14), AUD2HIGH (NR24), and AUD3HIGH (NR34)
  • (there is no AUD4HIGH!)

I find this harder to keep track of than just having "redundant" bit/flag constants named after each individual register. It's too easy to mistake that e.g. AUDLENF_DUTY would apply to AUD3LEN based on the name, or wonder why AUD3ENV is missing.

Also, we provide separate bit/flag constants for rIF and rIE, even though they both have the same correspondence of interrupts to bits.

I'd like to define individual constants, and deprecate (but still define) these.