integrate reachability map computation with export

[nikomatsakis]

Currently, we build up a reachability map as a pre-pass and then use this to avoid emitting metadata we don't have to emit. I was wondering if it would be possible instead to emit the metadata in a "DFS" fashion, wherein we never build a reachability map but instead just start by emitting metadata for exported/inlined things and then gradually emit metadata for other things that are referenced from those.

@marijnh does this seem plausible or is there an obvious reason it wouldn't work?

[graydon]

@nikomatsakis I can't tell if this is still valid but it seems non-critical for any particular milestone, just a nice cleanup.

[nikomatsakis]

I'm inclined to close this. I don't even know if it will work.