Throw more descriptive error when trying to serialize Livepack itself

[overlookmotel]

Trying to serialize Livepack's own internals throws error Function was not tracked.

e.g. serialize( serialize )

It would be better if Livepack recognised it's own internals and threw a more descriptive error Cannot serialize Livepack's internals.

The above case is silly. But it could happen accidentally if:

1. A function contains eval() and Livepack is in scope.
2. A class contains methods some of which are meant to run at build time, and some at runtime. The build-time methods might access Livepack.
3. Incorrect use of defer(), once that's implemented (Allow deferring import and evaluation to runtime #131).

[overlookmotel]

See also #97.