Pipeline builder for stdout trace exporter

[frigus02]

Part of #156

Implement the pipeline builder API for the stdout trace exporter, similar to the other exporters. With this a stdout trace pipeline can now be installed like this:

let tracer = opentelemetry::exporter::trace::stdout::new_pipeline().install();

I made 2 small changes along the way, which seemed to make sense to me. Happy to change in either direction, though. These are:

• Defer requirement of all additional writer traits to the PipelineBuilder::install function. Before Debug was required for the builder and Send + 'static were required only for the exporter. Now the builder is happy with only Write and requires Debug + Send + 'static for installation. I think I don't see any downsides here. We could also require all of these traits for the builder already. Not sure what's better.

• Defer wrapping writer in Mutex to when the exporter is created, which seems to be the point at which it's needed. Does this have any downsides?

Additional note:

This makes the pipeline the only way to construct a stdout exporter. This is in line with other exporters, but I'm not sure it's desired. Should we make Exporter::new public, so users can still create an exporter and setup a pipeline manually?

Update: Exporter::new is now public.

[jtescher]

This looks good @frigus02, I think making Exporter::new public is probably a good idea, can do that here or in a follow up PR

[frigus02]

Sounds good. I just pushed a commit to make Exporter::new public and add docs for it.