Trivial: whitespace.
Create a tesla-cat tool.

This tool merges several .tesla files into a single ManifestFile object within an output .tesla file.
Improve readability with typedef.
Drop some redundant code.
Additional const-ness.

Because we can and because it will be important when we work with top-level ManifestFile protobufs rather than locally-instantiated AutomatonDescription objects.
Use tesla-cat.

Use the ManifestFile protobuf representation within .tesla files rather than a series of AutomatonDescription protobufs that happen to be concatenated together with an arbitrary string ("===") in between them.