Introduce Storage::forget() to fix memory leak in Matrix::generic_resize()

[nickmertin]

Fixes #1378

Adds a trait method Storage::forget() to clean up the storage while forgetting (i.e., not dropping) the contents. Makes use of this method instead of mem::forget() in the implementations of Reallocator for DefaultAllocator.

[Andlon]

Thanks, this is great! I don't have the head space or time to wrap my head around the nuances of the Reallocator API here at the moment, but I wanted to ask if the documentation needs to be updated to reflect any possible changes in assumptions made. I don't fully understand them yet myself, I'd need a closer look later to try to verify.

[nickmertin]

Thanks! I looked over the documentation and I don't think any changes are necessary, as it maintains the assumption that the elements of the matrix are not dropped. My only question would be whether it would make more sense to move the new method to RawStorage instead of Storage.