Currently pybind11 doesn't check when you define a new object (e.g. a
class, function, or exception) that overwrites an existing one.  If the
thing being overwritten is a class, this leads to a segfault (because
pybind still thinks the type is defined, even though Python no longer
has the type).  In other cases this is harmless (e.g. replacing a
function with an exception), but even in that case it's most likely a
bug.

This code doesn't prevent you from actively doing something harmful,
like deliberately overwriting a previous definition, but detects
overwriting with a run-time error if it occurs in the standard
class/function/exception/def registration interfaces.

All of the additions are in non-template code; the result is actually a
tiny decrease in .so size compared to master without the new test code
(977304 to 977272 bytes), and about 4K higher with the new tests.