This diff modifies the engine package to make Experiment and
ExperimentBuilder interfaces rather than structs.
The previosuly existing structs are now named experiment{,Builder}.
This diff helps https://github.com/ooni/probe/issues/2184
because it allows us to write unit tests more easily.
There should be no functional change.
While there, I removed a bunch of deprecated functions, which were
unnecessarily complicate the implementation and could be easily
replaced by passing them a context.Context or context.Background().
97864b324f