prepl on Planck

It's very rough and mostly broken. But it kind of works as a proof of concept :)

Things I'm still trying to fix: (1) Buffering print statements and outputting them when they're flushed (otherwise prn results in separate outputs for the input and the new line). [This is a more complicated issue than I first realised. Arguably the example implementation in Clojure 1.10 is broken and the way my current implementation works is more correct.] (2) Removing indicator from warnings. (3) Removing prompt. (4) Outputting errors correctly. (5) Measuring the evaluation time (currently it just outputs zero). (6) Printing output values without wrapping them in strings.

@mike858 Cool! And, potentially quite useful!

Wow, prepl for Planck is a very exciting prospect.