[v3] use alias-declaration in <future>

[Jonathan Wakely]

This removes a TODO where I'd been waiting to use an alias-declaration
to make the code a bit simpler.

Also uses std::decay in the packaged_task constraint I added recently.

        * include/std/future (__future_base::_Ptr): Use alias-declaration.
        (__is_same_pkgdtask): Rename to __constrain_pkgdtask and use decay
        instead of remove_reference so that cv-quals are removed.

Tested x86_64-linux and x86_64-netbsd, committed to trunk.
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