[CM] Snd 21.4

[bil at ccrma.Stanford.EDU]

Snd 21.4:

nrepl: notcurses 2.1.6 or later is required now (notcurses got split
          into 2 libraries, notcurses-core and notcurses, and the APIs
          are not compatible.  notcurses-core is much smaller, so I'm
          using it).

checked: notcurses 2.3.0, Fedora 34, sbcl 2.1.4, Ubuntu 21.04

Thanks!: chai xiaoxiang, Kjetil Matheussen, Nick Black