Default :read hook for repl. Reads from *in* which must either be an
instance of LineNumberingPushbackReader or duplicate its behavior of both
supporting .unread and collapsing all of CR, LF, and CRLF into a single
\newline. repl-read:
- skips whitespace, then
- returns request-prompt on start of line, or
- returns request-exit on end of stream, or
- reads an object from the input stream, then
- skips the next input character if it's end of line, then
- returns the object.