When new marginal memory, concatenate with existing

[colin-grant-work]

With the addition of pagination, we are sometimes fetching memory redundantly. It would be ideal when loading new memory to just load the new memory and concatenate it with the old.

This is challenging, however, because the DAP allows adapters to return a partial result, so additional work would be required to determine how the new result corresponds to the old.

Originally posted by @colin-grant-work in #28 (comment)