Figure out some formal logging policy

[lihaoyi]

Our logging right now is a bit of a mess, we need to answer:

• How do we remove the verbosity of things like Running Core.test.upstreamCompileOutput, which hardly ever do any work?

• What goes to stdout and what goes to stderr?

• Where does --show fit into this, since a user calling --show is likely to want to capture the result as a clean JSON blob without all the misc log spam?

Ammonite's policy is that "misc" low-priority output (compiling files, blah) goes to stderr, leaving stdout for things that a user explicitly prints, so users can foo.sc > out.txt and be sure `out.txt doesn't contain any random trash.

[lihaoyi]

Fixed by fb68da0 and dd16d55