Integrate Fixnum class into Integer class

* The `Fixnum` constant is now an alias for the `Integer` class.
* Remove `struct mrb_state::fixnum_class` member.
  If necessary, use `struct mrb_state::integer_class` instead.

include/mruby/numeric.h

@@ -1,5 +1,5 @@
 /**
-** @file mruby/numeric.h - Numeric, Integer, Float, Fixnum class
+** @file mruby/numeric.h - Numeric, Integer, Float class
 **
 ** See Copyright Notice in mruby.h
 */
@@ -12,7 +12,7 @@
 /**
  * Numeric class and it's sub-classes.
  *
- * Integer, Float and Fixnum
+ * Integer and Float
  */
 MRB_BEGIN_DECL