In particular, encapsulate the state of the type table in a struct,
and add inline functions to initialize, free, and manipulate that
state. This replaces some ugly macros that made proper error handling
impossible.
While here, remove an unneeded test for NULL and a variable that is
initialized (many times!) but never used. The compiler didn't catch
these because of rampant use of the same variable to mean different
things in different places.
This commit should not cause any changes in functionality.
8e70682815