This reverts commit 31f4b63d.

It was a little hard for me to believe, but it seems that re-exporting
an impl doesn't work at a, because encoder::encode_info_for_mod requires
that all the impls in the current module's impl map be local (that is,
bound to a value in the current crate's item map). Fixed it.

Closes #2414.

Issue 2526 showed a test case where a library exported only a type
that was a synonym for a class. Because the class's destructor wasn't
getting marked as reachable, its linkage was wrongly getting set to
"internal". The solution is for reachability to traverse types.

Closes #2526.

This is useful on OS X in order to handle the .dSYM files.

Incoming

This lets us pass them to generic functions.

(basically a stripped-down version of comm, and a "driver" that
constructs a new port)

This allows destructors to be inlined, which is necessary since
classes can have both ty params and destructors.