Ruby

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.