Remove the obsolete term `Fixnum`.

Except for compatibility code.

doc/mruby3.md

@@ -77,8 +77,9 @@ to be default `mrb_value` representation. Now the default is
 
 Pack `mrb_value` in an `intptr_t` integer. Consumes less
 memory compared to `MRB_NO_BOXING` especially on 32-bit
-platforms. `Fixnum` size is 31 bits, so some integer values
-does not fit in `Fixnum` integers.
+platforms. Inlined integer size is 31 bits, so some `mrb_int`
+values does not fit in `mrb_value`. Those integers are allocated
+in the object heap as `struct RInteger`.
 
 ## `MRB_NAN_BOXING`
 

mrblib/range.rb

@@ -15,7 +15,7 @@ class Range
     val = self.begin
     last = self.end
 
-    if val.kind_of?(Fixnum) && last.nil?
+    if val.kind_of?(Integer) && last.nil?
       i = val
       while true
         block.call(i)

src/object.c

@@ -409,7 +409,7 @@ mrb_check_type(mrb_state *mrb, mrb_value x, enum mrb_vtype t)
     ename = "nil";
   }
   else if (mrb_integer_p(x)) {
-    ename = "Fixnum";
+    ename = "Integer";
   }
   else if (mrb_symbol_p(x)) {
     ename = "Symbol";