Add some debugging tools.

Compile with --cfg debug to use debug_malloc, and call debug_dump() to
print some stats.
Add bindings to the gc_typed.h header for precise-on-the-heap operation.

This requires runtime generated type information, and so is very slow;
it requires compiler assistance to be fast (and to be correct).
Add an example of the precise mode.
Directory structure.
Move the BoehmTraced impls out of tracing.rs, for neatness.
Update the README for tracing changes.