Use Xxx:is_smob instead of Xxx:unsmob when used in boolean contexts

Use Xxx:is_smob instead of Xxx:unsmob when used in boolean contexts

This saves a memory access and comparison and leads to better code
even when unsmob is called afterwards anyway.


Also contains commit:

Define inline members unsmob and is_smob for several derived classes

[dak, 2014-09-29 20:10:03]

A few oversights regarding unsmob->is_smob