Redesigned virtual call mechanism and user-facing syntax (breaking change!)

Sergey Lyskov pointed out that the trampoline mechanism used to override
virtual methods from within Python caused unnecessary overheads when
instantiating the original (i.e. non-extended) class.

This commit removes this inefficiency, but some syntax changes were
needed to achieve this. Projects using this features will need to make a
few changes:

In particular, the example below shows the old syntax to instantiate a
class with a trampoline:

class_<TrampolineClass>("MyClass")
    .alias<MyClass>()
    ....

This is what should be used now:

class_<MyClass, std::unique_ptr<MyClass, TrampolineClass>("MyClass")
    ....

Importantly, the trampoline class is now specified as the *third*
argument to the class_ template, and the alias<..>() call is gone. The
second argument with the unique pointer is simply the default holder
type used by pybind11.