symbol.c: add Symbol#name method

`Symbol#name` is similar to `#to_s` but returns a frozen string.

src/symbol.c

@@ -401,6 +401,30 @@ sym_to_s(mrb_state *mrb, mrb_value sym)
   return mrb_sym_str(mrb, mrb_symbol(sym));
 }
 
+/*
+ *  call-seq:
+ *     sym.name   -> string
+ *
+ *  Returns the name or string corresponding to <i>sym</i>. Unlike #to_s, the
+ *  returned string is frozen.
+ *
+ *     :fred.name         #=> "fred"
+ *     :fred.name.frozen? #=> true
+ */
+static mrb_value
+sym_name(mrb_state *mrb, mrb_value vsym)
+{
+  mrb_sym sym = mrb_symbol(vsym);
+  mrb_int len;
+  const char *name = mrb_sym_name_len(mrb, sym, &len);
+
+  mrb_assert(name != NULL);
+  if (SYMBOL_INLINE_P(sym)) {
+    return mrb_str_new_frozen(mrb, name, len);
+  }
+  return mrb_str_new_static_frozen(mrb, name, len);
+}
+
 /* 15.2.11.3.4  */
 /*
  * call-seq:
@@ -657,6 +681,7 @@ mrb_init_symbol(mrb_state *mrb)
   mrb_undef_class_method(mrb,  sym, "new");
 
   mrb_define_method(mrb, sym, "to_s",    sym_to_s,    MRB_ARGS_NONE());          /* 15.2.11.3.3 */
+  mrb_define_method(mrb, sym, "name",    sym_name,    MRB_ARGS_NONE());
   mrb_define_method(mrb, sym, "to_sym",  sym_to_sym,  MRB_ARGS_NONE());          /* 15.2.11.3.4 */
   mrb_define_method(mrb, sym, "inspect", sym_inspect, MRB_ARGS_NONE());          /* 15.2.11.3.5(x) */
   mrb_define_method(mrb, sym, "<=>",     sym_cmp,     MRB_ARGS_REQ(1));