add sym_validate_len() to validate symbol length

This also fixes an off-by-one in mrb_check_intern().
use RITE_LV_NULL_MARK for better readability

The comment didn't say why the length is reserved.
Using RITE_LV_NULL_MARK makes things a bit clearer.
delete obsolete macros (mrb_basic + mrb_object)

Over a year ago is not "soon" anymore. See:
88483fad0036e5cb023940f7c6d18006517fa731
243669308dfd10c8444c2992a883e165b9209941
Merge pull request #2576 from cremno/correctly-validate-symbol-length

correctly validate symbol length
Merge pull request #2577 from cremno/remove-obsolete-macros

delete obsolete macros (mrb_basic + mrb_object)
remove unnecessary MRB_API from mrb_num_div(); close #2578