Remove @-boxes from the visitor

[brson]

Using the GC in the visitor means that there are contexts where logging doesn't work correctly. It also forces tasks to use GC that otherwise wouldn't want to.

Partly blocking #3309

[brson]

Nominating feature complete

[graydon]

accepted for backwards-compatible milestone

[emberian]

Visiting for triage; nothing to add

[alexcrichton]

This has since been done. The visitor now primarily uses traits, and it's up to the implementors of the traits to decide whether to use @-boxes or not (as it should be).

Nice work everyone!