Set maximum string (and symbol) size to 65534 (`UINT16_MAX-1`).

The previous value (`UINT16_MAX`) was too long for symbols, so it raises
an exception after the length check.

include/mruby/compile.h

@@ -105,7 +105,7 @@ struct mrb_parser_heredoc_info {
   mrb_ast_node *doc;
 };
 
-#define MRB_PARSER_TOKBUF_MAX 65536
+#define MRB_PARSER_TOKBUF_MAX (UINT16_MAX-1)
 #define MRB_PARSER_TOKBUF_SIZE 256
 
 /* parser structure */