crux: demo ring buffer

[skhal]

Demo a ring buffer: a buffer of fixed length N, where the tail index rotates in the range [0, N).

Ref: https://cs.opensource.google/go/go/+/master:src/cmd/vendor/golang.org/x/term/terminal.go;l=991-1000;drc=ca5ffe0092363f21df2c57b50144ba056f260049