Enum iterators now return the same as struct iterators. Also docs.
Cleaned up comments.
Prevent nested includes of ANY datatype header from a datafile.

More specifically, the X{type}_H_NESTED_ macros have all been replaced
with XDATA_NESTED_. The reasoning behind this is that one datafile could
conceivably include multiple (possibly custom-written) datatype headers.
Added enum XASSOC option. Needs examples in xenum.h comments.
Typo
Fixed example.c bug, added -std=c99 to Makefile.