cmd/cue: add cue exp gengotypes

This is a first version of an experimental feature to generate
Go types from CUE definitions, a sort of opposite to `cue get go`.

The tool and added test suite are complete enough to the point that
we can start using the tool ourselves in places where we were using
`cue get go` but would have preferred the CUE as a source of truth.

There are plenty of TODOs left here, and surely a number of bugs too,
but this is a good starting point for a review.
Incremental changes can be made afterwards, which are easier to review.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: Ifd21c87196643fdb44900f9258493b67d614e11a
Dispatch-Trailer: {"type":"trybot","CL":1206583,"patchset":2,"ref":"refs/changes/83/1206583/2","targetBranch":"master"}