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Allows checking the Python types before creating an object instead of
after. For example:
```c++
auto l = list(ptr, true);
if (l.check())
   // ...
```
The above is replaced with:
```c++
if (isinstance<list>(ptr)) {
    auto l = reinterpret_borrow(ptr);
    // ...
}
```

This deprecates `py::object::check()`. `py::isinstance()` covers the
same use case, but it can also check for user-defined types:
```c++
class Pet { ... };
py::class_<Pet>(...);

m.def("is_pet", [](py::object obj) {
    return py::isinstance<Pet>(obj); // works as expected
});
```

The current inheritance testing isn't sufficient to detect a cache
failure; the test added here breaks PR #390, which caches the
run-time-determined return type the first time a function is called,
then reuses that cached type even though the run-time type could be
different for a future call.

This allows a slightly cleaner base type specification of:

    py::class_<Type, Base>("Type")

as an alternative to

    py::class_<Type>("Type", py::base<Base>())

As with the other template parameters, the order relative to the holder
or trampoline types doesn't matter.

This also includes a compile-time assertion failure if attempting to
specify more than one base class (but is easily extendible to support
multiple inheritance, someday, by updating the class_selector::set_bases
function to set multiple bases).

Use simple asserts and pytest's powerful introspection to make testing
simpler. This merges the old .py/.ref file pairs into simple .py files
where the expected values are right next to the code being tested.

This commit does not touch the C++ part of the code and replicates the
Python tests exactly like the old .ref-file-based approach.