Fix NoMethodError in TypeProf::Core::Type::Singleton

I encountered the following error:

```
/Users/sinsoku/ghq/github.com/ruby/typeprof/lib/typeprof/core/ast/sig_type.rb:10:in `block in typecheck_for_module': undefined method `args' for an instance of TypeProf::Core::Type::Singleton (NoMethodError)

            f_args.zip(ty.args) do |f_arg_node, a_arg_ty|
                         ^^^^^
        from /Users/sinsoku/ghq/github.com/ruby/typeprof/lib/typeprof/core/graph/vertex.rb:11:in `each_key'
        from /Users/sinsoku/ghq/github.com/ruby/typeprof/lib/typeprof/core/graph/vertex.rb:11:in `each_type'
        from /Users/sinsoku/ghq/github.com/ruby/typeprof/lib/typeprof/core/ast/sig_type.rb:5:in `typecheck_for_module'
        from /Users/sinsoku/ghq/github.com/ruby/typeprof/lib/typeprof/core/ast/sig_type.rb:669:in `typecheck'
        from /Users/sinsoku/ghq/github.com/ruby/typeprof/lib/typeprof/core/ast/sig_type.rb:483:in `block in typecheck'
        from /Users/sinsoku/ghq/github.com/ruby/typeprof/lib/typeprof/core/ast/sig_type.rb:482:in `each'
        from /Users/sinsoku/ghq/github.com/ruby/typeprof/lib/typeprof/core/ast/sig_type.rb:482:in `typecheck'
```

The `args` method is only defined on `Type::Instance`, so it
adds a conditional expression to avoid the error.

Author: sinsoku

lib/typeprof/core/ast/sig_type.rb

@@ -5,7 +5,7 @@ def self.typecheck_for_module(genv, changes, f_mod, f_args, a_vtx, subst)
       a_vtx.each_type do |ty|
         ty = ty.base_type(genv)
         while ty
-          if ty.mod == f_mod
+          if ty.mod == f_mod && ty.is_a?(Type::Instance)
             args_all_match = true
             f_args.zip(ty.args) do |f_arg_node, a_arg_ty|
               unless f_arg_node.typecheck(genv, changes, a_arg_ty, subst)

scenario/regressions/singleton-is-a-module.rb

@@ -0,0 +1,8 @@
+## update
+class C
+  def self.foo=(v)
+    v.is_a?(Module)
+  end
+
+  self.foo = 1
+end