Progress reporting for initial preprocessing

[wlandau]

Million-target pipelines take a long time to initialize. To the user, tar_make() on an existing behemoth pipeline appears to hang when it starts. It isn't obvious that necessary (and highly optimized) preprocessing steps are happening under the hood. Maybe targets should print extra messages to the R console.

[wlandau]

Converting to a discussion. I have a lot of recency bias from my recent optimization work. Planning to come back to it later.

[koefoeden]

I think this is a good idea - good be something as simple as a single message saying "Initializing... (could take while for large pipelines) " or similar. A completely blank new line is always a bit unsettling if if's there for too long!

[wlandau]

Done in #1482