Add "fallthrough" keyword, require a flow statement to end case blocks.

Case blocks in switch statements now must end in a break, return, or
fallthrough statement to give best mix of safety, readability, and
flexibility.

The new fallthrough keyword explicitly allows control to be passed to the
next case block in a switch statement.

Addresses #754.

src/scan.l

@@ -282,6 +282,7 @@ else	return TOK_ELSE;
 enum	return TOK_ENUM;
 event	return TOK_EVENT;
 export	return TOK_EXPORT;
+fallthrough	return TOK_FALLTHROUGH;
 file	return TOK_FILE;
 for	return TOK_FOR;
 function	return TOK_FUNCTION;