Extract tesla_key function(s) into key.c.
Add more convenience typedefs in internal header.
Add tesla_key_union().

This computes the union of two keys, storing the result in the first argument.
Use a more succinct Automaton description.
Trivial: fix an incorrect comment.
Remove TODO: error handling at assertion sites.

We already do sensible-ish error handling at other instrumentation sites; let's incorporate it at assertion sites, too.