convert shape code to be visitor glue

[nikomatsakis]

I would like to do away with the shape code and instead have the compiler generator "visitor glue", which is glue code that walks the data structure and invokes methods on a visitor. The methods would be encoded as a struct of functions. This could replace the logging, universal equality, and other functions of glue code. If it's fast enough, we may be able to replace the take, drop, and free glue with visitor glue, but if not, we could at least tie the code generation together so that they are all driven from the same codebase.

[pcwalton]

There are three facets to this: equality, logging, and cycle collection. For comparison, shape code is currently used only for != and pattern matching of strings. For logging and cycle collection, it's used everywhere.

The fix for comparison is to add ne to the Eq trait and to make pattern matching of strings call into a magic string comparison lang-item.

The fix for logging is to make logging call into a magic lang-item that uses the visitor glue. (It should eventually be a macro.)

The fix for cycle collection is to either replicate the cycle collector with visitor glue or to switch to a conservative garbage collector for @ boxes.

[nikomatsakis]

Deferring to 0.5

[catamorphism]

As far as I know, this is done.