`parse_lines(str, line_delim)`

It seems like at least a couple different things (the repl, the studio ide) independently take user input, break it into lines, and then massage those into expressions.

Unfortunately, we can't quite use `parse(str, pos)` in a loop because line breaks *might* signal the end of an expression (and using `parse` we'll just get a `ParseError("extra token after end of expression")`)