Nsinit executor

[kofalt]

Early days, so a few things are bundled:

• A submodule update to our minor fork of errors to be tolerant of mundane-typed panics.
• Dispatch refactored into packages such that an executor can use them without an import cycle.
• MVP implementation of input/tar and output/tar, using exec wrapping and blind to hashes.
• A very basic nsinit executor. Assumes the existence of a docker0 bridge for outbound networking, does not ensure the existence of a valid DNS configuration, exec wraps nsinit... but works great!

Unresolved:

• The executor will create a folder for each output's location. This is great for scripts that assume the existence of an output folder (so basically, every script every written :/ ) but won't be great if an output is supposed to be a single file.
• The output tar transport is kinda wacky with its transform call, and both tar transports can be replaced later with native usage of treewalk / etc.
• The Output interface method I added currently takes the rootFS path, and is expected to join the output's Location to get something real. This is largely an artefact of our earlier internal conversation + confusion, and I could be easily convinced to bring it in line with the Input interface.

[warpfork]

Lots of stuff that's in placeholder territory, but this gets us rolling :+1:

[kofalt]

Pushed fixes; merge when CI green