Refactor `Memory` to reduce unsafe code

[eduardosm]

Refactor Memory to avoid dealing with manual allocations, raw pointers and memory layouts.

Now it uses a Vec (with length zero and desired capacity) and Vec::spare_capacity_mut to access the allocated capacity.

[cmpute]

Thanks for the PR! What necessitates this change in your opinion? I think the original author of the memory allocation utilities (i.e tczajka) keeps this design because it will be easier to create customized allocation logics in future.

[eduardosm]

Since it was only used to allocate slices of Word, I simplified it accordingly. Is there a need to allocate arbitrary layouts?

[cmpute]

This change might be related to #43 , lets wait for that issue to have a rough idea of how to improve this.