Allow Multi-Line Inputs In REPL

This commit message has been written with the new multi-line support that is now
implemented in the REPL. There is a new specialized QuoteTerminator structure
that provides the means to handle this by checking if the last character provided
was a '\' and trimming the next line, or keeping state of wether a single or
double quote has been terminated, and continuing to buffer until both are
properly terminated.