Move `Intefer#chr` to `Integral#chr`.

Since mruby mixes `Integer` and `Float`, integer operations have
been moved to `Integral` module.

mrbgems/mruby-numeric-ext/src/numeric_ext.c

 #include <limits.h>
 #include <mruby.h>
 
+static inline mrb_int
+to_int(mrb_value x)
+{
+  double f;
+
+  if (mrb_fixnum_p(x)) return mrb_fixnum(x);
+  f = mrb_float(x);
+  return (mrb_int)f;
+}
+
+/*
+ *  Document-method: Integer#chr
+ *  call-seq:
+ *     int.chr  ->  string
+ *
+ *  Returns a string containing the character represented by the +int+'s value
+ *  according to +encoding+.
+ *
+ *     65.chr    #=> "A"
+ *     230.chr   #=> "\xE6"
+ */
 static mrb_value
 mrb_int_chr(mrb_state *mrb, mrb_value x)
 {
   mrb_int chr;
   char c;
 
-  chr = mrb_fixnum(x);
+  chr = to_int(x);
   if (chr >= (1 << CHAR_BIT)) {
     mrb_raisef(mrb, E_RANGE_ERROR, "%S out of char range", x);
   }
@@ -19,7 +40,7 @@ mrb_int_chr(mrb_state *mrb, mrb_value x)
 void
 mrb_mruby_numeric_ext_gem_init(mrb_state* mrb)
 {
-  struct RClass *i = mrb_class_get(mrb, "Integer");
+  struct RClass *i = mrb_module_get(mrb, "Integral");
 
   mrb_define_method(mrb, i, "chr", mrb_int_chr, MRB_ARGS_NONE());
 }