Check method existence for Enumerators; fix #3920

The issue #3920 was fixed but the fundamental flaw of lack of stack
depth check along with fibers still remains, even though it's not
easy to cause the issue.  Use `MRB_GC_FIXED_ARENA` to avoid the issue
for workaround.

After this patch, `obj.to_enum` raises `ArgumentError` if the object
does not respond to the enumerating method. This is incompatible to
CRuby but I think this behavior is better and CRuby should be updated
to behave like this.

mrbgems/mruby-enum-lazy/mrblib/lazy.rb

@@ -44,7 +44,7 @@ class Enumerator
 
     def to_enum(meth=:each, *args, &block)
       unless self.respond_to?(meth)
-        raise NoMethodError, "undefined method #{meth}"
+        raise ArgumentError, "undefined method #{meth}"
       end
       lz = Lazy.new(self, &block)
       lz.obj = self

mrbgems/mruby-enumerator/mrblib/enumerator.rb

@@ -614,6 +614,9 @@ module Kernel
   #     enum.first(4) # => [1, 1, 1, 2]
   #
   def to_enum(meth=:each, *args)
+    unless self.respond_to?(meth)
+      raise ArgumentError, "undefined method #{meth}"
+    end
     Enumerator.new self, meth, *args
   end
   alias enum_for to_enum

mrbgems/mruby-enumerator/test/enumerator.rb

@@ -22,8 +22,7 @@ assert 'Enumerator.new' do
   assert_equal [1,2,3], Enumerator.new(@obj, :foo, 1,2,3).to_a
   assert_equal [1,2,3], Enumerator.new { |y| i = 0; loop { y << (i += 1) } }.take(3)
   assert_raise(ArgumentError) { Enumerator.new }
-  enum = @obj.to_enum
-  assert_raise(NoMethodError) { enum.each {} }
+  assert_raise(ArgumentError) { @obj.to_enum }
 
   # examples
   fib = Enumerator.new do |y|