Alignment constraints on `Storable`/`alloca`

• It's unclear if the alignment value of a Storeable instance must be a power of two. (Probably should)
• Currently alloca does not mention any alignment requirements or guarantess, but ends up allocating aligned memory. (Which is reasonable).

Ideas:

• Add preconditions to alloca in regards to the storable instance it ends up using.
• Or alloca could round up to the nearest power of two.
• In regards to Storeable we probably just want to add a requirement on the alignment to be a power of two.