Fix NoMethodError in SigTyTupleNode#typecheck for instance types

[sinsoku]

SigTyTupleNode#typecheck was passing wrong parameter (ty instead of vtx)
to f_ty.typecheck when handling Type::Instance objects, causing:
"undefined method `each_type' for an instance of TypeProf::Core::Type::Instance".

/Users/sinsoku/ghq/github.com/ruby/typeprof/lib/typeprof/core/ast/sig_type.rb:5:in `typecheck_for_module': undefined method `each_type' for an instance of TypeProf::Core::Type::Instance (NoMethodError)

      a_vtx.each_type do |ty|
           ^^^^^^^^^^
        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:727:in `block (2 levels) in typecheck'
        from /Users/sinsoku/ghq/github.com/ruby/typeprof/lib/typeprof/core/ast/sig_type.rb:726:in `each'
        from /Users/sinsoku/ghq/github.com/ruby/typeprof/lib/typeprof/core/ast/sig_type.rb:726:in `block in typecheck'

[sinsoku]

I was able to create a scenario that reproduced the issue, so I force-pushed it.

I found this issue while analyzing files in Redmine v6.10.
The file is at https://github.com/redmine/redmine/blob/6.1.0/app/models/mail_handler.rb#L425.

The steps to reproduce are as follows:

$ git clone --depth 1 --branch 6.1.0 https://github.com/redmine/redmine.git tmp/redmine
$ bin/typeprof tmp/redmine/app/models/mail_handler.rb
# TypeProf 0.30.1

[0/1] tmp/redmine/app/models/mail_handler.rb/Users/sinsoku/ghq/github.com/ruby/typeprof/lib/typeprof/core/ast/sig_type.rb:5:in 'TypeProf::Core::AST.typecheck_for_module': undefined method 'each_type' for an instance of TypeProf::Core::Type::Instance (NoMethodError)

      a_vtx.each_type do |ty|
           ^^^^^^^^^^
        from /Users/sinsoku/ghq/github.com/ruby/typeprof/lib/typeprof/core/ast/sig_type.rb:656:in 'TypeProf::Core::AST::SigTyInstanceNode#typecheck'
        from /Users/sinsoku/ghq/github.com/ruby/typeprof/lib/typeprof/core/ast/sig_type.rb:714:in 'block (2 levels) in TypeProf::Core::AST::SigTyTupleNode#typecheck'

[mame]

🙈