extconf: add have_func checks for `rb_sym2str()` and `rb_str2sym()`
rb-grn.h: add backward compatible macro for `rb_sym2str()` and `rb_strsym()`
use rb_sym2str() and instead of rb_id2name(SYM2ID()).
use `rb_str2stm()` instead of `ID2SYM(rb_intern())`