Allow resizing scratch region

Currently, the size of the scratch region is set when a snapshot is first created from a binary, and it can never be changed. This is unfortunate, because many workloads use a lot of memory during an early initialisation phase, and then very little memory in repeated runtime phases that occur after another snapshot. It would be nice to offer some API that changed the size of the scratch region on a snapshot so that we could avoid this situation, since the size of the scratch region is directly linked to performance, especially on certain kernel/hypervisor combinations.

The issue names no files, tests, or entry points. Start by locating snapshot creation and scratch-region sizing in the Rust code, then determine the API and lifecycle constraints for resizing. Done means a supported way to change a snapshot's scratch-region size without recreating it, with behavior validated for the described initialization and runtime phases.

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