Make lastline and nextline to be rb_parser_string

This commit changes `struct parser_params` lastline and nextline
from `VALUE` (String object) to `rb_parser_string_t *` so that
dependency on Ruby Object is reduced.
`parser_string_buffer_t string_buffer` is added to `struct parser_params`
to manage `rb_parser_string_t` pointers of each line. All allocated line
strings are freed in `rb_ruby_parser_free`.

internal/ruby_parser.h

@@ -72,6 +72,10 @@ enum lex_state_e {
     EXPR_NONE = 0
 };
 
+RUBY_SYMBOL_EXPORT_BEGIN
+VALUE rb_str_new_parser_string(rb_parser_string_t *str);
+RUBY_SYMBOL_EXPORT_END
+
 VALUE rb_node_sym_string_val(const NODE *);
 VALUE rb_node_line_lineno_val(const NODE *);
 VALUE rb_node_file_path_val(const NODE *);