Commentary
Small amount of additional commentary
Support building one SSA DexFile on top of another

This lets us serialize a big chunk of dalvik that is shared between
applications and then deserialize it later while loading an application.
Version bump for API breakage
Update tests to the new toSSA api
Add a file header to the binary format

It includes identifying information and a version number.
Unfortunately, bumping the version number will be a manual process.
New tool options
Start a big test for the serialization code
Don't overwrite fields while tying the knot.
Fix method key generation

We were using the wrong keys for some kinds of method definitions, which
meant that they could not be looked up successfully at call sites.
Flesh out the tests a lot more
Roundtrip tests