subscriber: make Compact event formatter not terrible

[hawkw]

Depends on #1067

Motivation

Currently, the Compact event formatter in tracing-subscriber
is...kind of bad, and nobody seems to use it. It outputs the names of
all the spans in the current context, and (inexplicably) the names of
all the fields on the current span, but not their values. This isn't
very useful.

Solution

This branch rewrites the Compact formatter to be a little less bad.
Now, we output all the fields from the current span context, but skip
span names, and we shorten the level to a single character when it's
enabled. Additionally, using the compact formatter disables targets by
default. Now, the lines are nicely short, but maybe still useful.

Example output before this change:

...and after: