gremlin/gremlin.in: Walk the various output trees concurrently.

Rather than doing them one after the other.  This makes the progress
report rather more helpful, and may even make things slightly faster.

The change isn't as complicated as the diff makes it look.  The Grobbler
class has been replaced with a function, with its methods now nested
functions within it, which actually makes calling it slightly simpler.
Another simplification is that the function takes `TargetJob' objects
rather than directory names, simply because they're a convenient way to
bundle the directory names with their conversion policies.  But this
means that the function has been sunk beneath the various auxiliary
class definitions used by the top-level config parser, and Git diff
makes a mess of this.