Though rb_enc_isalnum is encoding aware function, its argument here
  is *m, which is a single byte. Therefore ISDIGIT is faster.

* symbol.c (is_special_global_name): ditto.

* symbol.c (rb_enc_symname_type): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55178 b2dd03c8-39d4-4d8f-98ff-823fe69b080e