Now, instead of being a black box that you shovel strings into and
eventually extract a final answer, it exposes enough structure fields
to the world that you can append things to it _and_ look inside its
current contents. For convenience, it exports its internal pointer as
both a char * and an unsigned char *.
This centralises a few things that multiple header files were
previously defining, and were protecting against each other's
redefinition with ifdefs - small things like structs and typedefs. Now
all those things are in a defs.h which is by definition safe to
include _first_ (out of all the codebase-local headers) and only need
to be defined once.
