break lexgen cborgen circular dependence

[brianolson]

OLD: When a new type comes in through lexgen, it cannot be compiled until after cborgen runs, but cborgen cannot run because the type cannot be compiled. So comment out a section, run cborgen, then uncomment the hidden code.

New: make lexgen; make cborgen; make test done
A type check that used to be done at compile time (which would fail) is now done in unit test code, which will pass after cborgen is run.

Requires forthcoming cbor-gen PR whyrusleeping/cbor-gen#102

[jazware]

Looks pretty good to me, I'm interested in testing it out a bit to make sure we don't regress here though so I might pull this version into some of my code that gets more hands-on with types that require CBOR marshalling/unmarshalling.

I'd wait for a @whyrusleeping approval on these changes though since I'm not 100% sure on the cbor dependency bits.

[brianolson]

this doesn't handle a case I ran into later: lexgen for some types writes MarshalCBOR/UnmarshalCBOR if all their fields are objects known to lexgen. these implementations don't compile and still prevent cbor-gen from running and they need to be temporarily commented out. :-/

My other guess is that we split sources and use build flags. e.g. write Foo struct to foo.go and foo_cborgen.go flagged with //go:build !cborgen so that we can run cborgen with -tags cborgen and hide those sources during cborgen run.