XS fxGrowKeys should delegate to a new fxAllocateKeys, track with a meter

[warner]

What is the Problem Being Solved?

Per agoric-labs/moddable#9 (comment) :

fxGrowSlots uses c_realloc() directly, unlike slots/chunks which go through an intermediate fxAllocateChunks/fxAllocateSlots, which xsnapPlatform.c overrides to track in the space meter. So the space used by keys won't be tracked.

This keyArray wants to use re-alloc, which is not quite as easy to wrap/track. But when we start trying to meter space allocation in earnest, we should consider creating an fxReallocKeys(), which can be controlled by the platform, and which can compare the old size (i.e. it can somehow learn the old size, maybe it should accept a third arg with the old size, maybe the->keyCount?) and new size, and apply the difference to the meter.

Description of the Design

Security Considerations

Scaling Considerations

Test Plan