add an nfa_size macro to determine the size (in bytes) of an Nfa object
check for NFA_NDEBUG as well as NDEBUG

This allows libnfa's assertions to be disabled without disabling all other
assertions in a program.
remove the NFA_IMPLEMENTATION thing

It's not really worth it; it's easier and clearer to explicitly
#include "nfa.c" in your own source file if you want to embed the source.
add a couple of todo items
minor wording change in nfa.h comment (to match terminology from docs)
nfa_builder_output_size: set the error state if an error is detected
nfa_builder_output: check for errors after calling nfa_builder_output_size
start writing documentation
rename Documentation -> manual