While working with the runtime `Memory` object, it became clear that
some refactoring was needed. In order to implement shared memory from
the threads proposal, we must be able to atomically change the memory
size. Previously, the split into variants, `Memory::Static` and
`Memory::Dynamic`, made any attempt to lock forced us to duplicate logic
in various places.
This change moves `enum Memory { Static..., Dynamic... }` to simply
`struct Memory(Box<dyn RuntimeLinearMemory>)`. A new type,
`ExternalMemory`, takes the place of `Memory::Static` and also
implements the `RuntimeLinearMemory` trait, allowing `Memory` to contain
the same two options as before: `MmapMemory` for `Memory::Dynamic` and
`ExternalMemory` for `Memory::Static`. To interface with the
`PoolingAllocator`, this change also required the ability to downcast to
the internal representation.
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