Add a deinitialize method to the `Alloc` trait

[nikomatsakis]

@huonw pointed out that types like Vec that peddle in deinitialized memory must have a way to eliminate stray references. The allocator can initialize memory when it is given out initially, but can't handle the case where data was pushed and then popped.

cc @pnkfelix

[pnkfelix]

hmm. This could have been handled by the old vector design (where the heap-allocated object carried the length and capacity), but that ship has sailed.

So yes, it seems like types like Vec need to invoke some sort of callback on the allocator (which may then just zero out the memory, or store the info in some other metadata structure somewhere, or ignore it entirely in the case of a conservative scheme ... assuming the the latter is actually an option).