REPL

[loganzartman]

Given more backend support like cursor save/restore and relative cursor motion, we could create a rich REPL utility for the non-alternate screen. Some ideas:

• REPL helper which can print() a prompt, an input line, and extra text below
• a LineBuffer, which is an auto-resizing Nx1 buffer
  • implements blit_to() to support copying (with wrapping) to another Buffer
  • implements render to backend without cursor movement, allowing automatic wrapping
• REPL accepts LineBuffer for prompt and input line (maybe these are both a single LineBuffer) and an "extra" or "hint" buffer
• hint buffer can be displayed below the input line, either transiently or permanently (creating more space before the next prompt/input line rather than being erased)
• rendering of hint buffer would require using relative cursor motion--can we extend this to all buffer rendering or do we need a separate renderer for REPL?