Add configurable char flushing at REPL end of lines

[jpellegrini]

Hi @egallesio !
I'm not sure my implementation is ok, but as far as I have tested, it seems good...

This was inspired by a recent change in Gambit.

When a line is intered int the REPL, STklos will flush the characters in the list contained in the parameter obejct repl-flushed-characters. The default value is (#\space #\tab). Newlines and end-of-file are always flushed.

 stklos> (read-char)      <= no character after the closing parenthesis
 4                        <= user input
 #\4                      <= return value
 stklos> (read-char)      <= a space after the closing parenthesis
 4                        <= user input
 #\4                      <= return value
 stklos> (read-char)3     <= '3' after closing parenthesis
 #\3                      <= return value
 stklos> (read-char) 3    <= space and '3' after closing parenthesis
 #\3                      <= return value
 stklos> (read-char)35    <= '35' after closing parenthesis
 #\3                      <= return value
 5                        <= next return value

stklos>  (repl-flushed-characters '())
 stklos> (read-char)      <= no character after the closing parenthesis
 4                        <= user input
 #\4                      <= return value
 stklos> (read-char)      <= a space after the closing parenthesis
 #\space                  <= return value (the space was not skipped)
 stklos> (read-char)3     <= '3' after closing parenthesis
 #\3                      <= return value
 stklos> (read-char) 3    <= space and '3' after closing parenthesis
 #\space                  <= return value (the space was not skipped)
 #\3                      <= next return value
 stklos> (read-char)35    <= '35' after closing parenthesis
 #\3                      <= return value
 5                        <= next return value

[jpellegrini]

This is interesting so we can enter characters to read-char on the REPL (currently STklos won't let us type, it will read the newline and return it)