Refactor deconstruct to use less protobuf

[glennhickey]

Changelog Entry

To be copied to the draft changelog by merger:

• vg deconstruct now does path-based (formerly -e) deconstruction by default. Old default behaviour of exhaustively processing (tiny) sites is deprecated.
• if -a is not used, vg deconstruct will recursive on child snarls of snarls it failed to process (like vg call) - functionality that was, I think, dropped a while back.

Description

This refactor

• makes the code a bit more modular
• cuts out everything protobuf except for snarl enumeration. which is now confined to a little function. so upgrading to the new distance-index-based format is now pretty trivial, but probably doesn't make sense until other tools like call are ready...
• the exhaustive traversal support complicates the logic a bunch and is very proto dependent and I don't think used in practice, so I dropped it. if we really need something like this, I think a better approach would be going through some kind of path cover then doing a path deconstruction.

The whole point of doing this is to add better nesting support, but my branch for that is a mess so I'm breaking it into smaller PRs...