Remove autoImpl and return more from autoSpec

[tophtucker]

To support compiling an autoSpec to the equivalent source using explicit marks. This expands the returned surface area of autoSpec a lot; so far we’ve been treating it as an undocumented and unstable function for mostly internal use, so maybe that’s OK.

The giant switch statements could be rewritten to something more like:

const MARKS = {
  barX,
  barY…
}

// in auto
markImpl = MARKS[markImpl]

but I seem to remember you saying you’d prefer the explicitness of a switch.

[mbostock]

I thought of a way to avoid the verbosity of the switch statements, and to retain the symbolic references internally (and minimize the diff). This removes a little bit of error checking, but I don’t think we really needed that anyway since the names are only generated internally and we already have unit tests for it.