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.h

@@ -248,7 +248,7 @@ typedef struct mrb_state {
 #ifndef MRB_NO_FLOAT
   struct RClass *float_class;
 #endif
-  struct RClass *fixnum_class;
+  struct RClass *integer_class;
   struct RClass *true_class;
   struct RClass *false_class;
   struct RClass *nil_class;
@@ -890,8 +890,8 @@ MRB_API struct RClass* mrb_define_module_under_id(mrb_state *mrb, struct RClass
  * | `s`  | {String}       | const char *, {mrb_int} | Receive two arguments; `s!` gives (`NULL`,`0`) for `nil` |
  * | `z`  | {String}       | const char *      | `NULL` terminated string; `z!` gives `NULL` for `nil` |
  * | `a`  | {Array}        | {mrb_value} *, {mrb_int} | Receive two arguments; `a!` gives (`NULL`,`0`) for `nil` |
- * | `f`  | {Fixnum}/{Float} | {mrb_float}       |                                                    |
- * | `i`  | {Fixnum}/{Float} | {mrb_int}         |                                                    |
+ * | `f`  | {Integer}/{Float} | {mrb_float}    |                                                    |
+ * | `i`  | {Integer}/{Float} | {mrb_int}      |                                                    |
  * | `b`  | boolean        | {mrb_bool}        |                                                    |
  * | `n`  | {String}/{Symbol} | {mrb_sym}         |                                                    |
  * | `d`  | data           | void *, {mrb_data_type} const | 2nd argument will be used to check data type so it won't be modified; when `!` follows, the value may be `nil` |

include/mruby/class.h

@@ -36,7 +36,7 @@ mrb_class(mrb_state *mrb, mrb_value v)
   case MRB_TT_SYMBOL:
     return mrb->symbol_class;
   case MRB_TT_FIXNUM:
-    return mrb->fixnum_class;
+    return mrb->integer_class;
 #ifndef MRB_NO_FLOAT
   case MRB_TT_FLOAT:
     return mrb->float_class;

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