Want builder-consuming build methods

@nick writes in !474 (comment 2800482)

Incidentally this clone makes me with that derive_builder also created a "consuming build" method, for cases where we can destroy the builder and move its members into the build object.

I agree. I think options are:

1. Have derive_builder provide build_consuming alongside build. (I think .build() would be .clone().build_consuming() internally.) Name TBD.
2. Change our build to consume and ask people who want that to do .clone().build(). This would break BlahBuilder::default().some_param(42).build() because the some_param setter returns only a borrow of the builder.

I considered the following but ruled them out:

3. Change our use of derive_builder to request the "owned" pattern (doesn't work with sub-builders, since the sub-builder accessor must return &mut)
4. Do something different for different structs (the inconsistency would be pretty awful).

I'm quite a fan of 2, actually. But it would be an API break.