Lifetime issue in `tech_library` constructor

[rocallahan]

The tech_library constructor initializes _supergates_spec to be a reference to the super_lib referenced by the supergates_spec parameter. But the default value for that parameter is a default-constructed super_lib that will be destroyed when the tech_lib constructor returns. So if this default is used _supergates_spec becomes a dangling reference.

(Not super urgent since we haven't actually hit this yet, but it looks like a problem.)

[aletempiac]

Thank you for reporting! This issue shouldn't affect the execution of any program. However, it is good to remove it. Let me know if the change in #665 is good enough.

[rocallahan]

Yes, thanks.