Context note operations using `_` prefix

[atsushieno]

After adding Bn operation, I figured that it is still not very user-friendly. How can we do better?

Idea:

• _B= operation, as well as _B_ operations to take immediately preceding note context
  • e.g. BEND_CENT_MODE24 c0_B_0,4,0,1200 e4_B_0,4,0,-1200 (per-note pitchbend on both c and e, where c gets pitch goes up, while e gets pitch goes down.

[atsushieno]

Context note is now stored as __latest_note variable, and we make use of it in Bc= macro.