Finish metadata::encoder::encode_info_for_mod

[catamorphism]

encode_info_for_mod seems to encode whether each module child is exported, but it only does that for impls and structs. So I guess traits all get exported by default? It's unclear to me what needs to be changed, if anything (a FIXME, formerly XXX, just says "Encode these two" for the default case).

[catamorphism]

Nominating for milestone 5 (production-ready).

[graydon]

metadata encoding needs quite a bit of work. put this in a metabug associated with it, and accept for production ready.

[metajack]

revisiting for triage; nothing to add

[pnkfelix]

We assume this has been done. Closing until someone can show its actually broken with concrete example..