Wasmtime: Preemption and traps during bulk operations enable breaking internal VM state

### Impact Wasmtime's implementation of bulk-data-transfer WebAssembly instructions, such as `memory.copy`, contains a vulnerability when a preemption via epochs or fuel is combined with altering the store's state or cancelling a computation. To prevent these operations from taking too long Wasmtime injects fuel/epoch checks during these operations, but this enables embedders, and possible WebAssembly, to witness intermediate state in the middle of the operation. Examples of this include: * When a non-nullable WebAssembly table is grown the new elements initially start as null and are filled in as part of a loop with preemption checks. If this computation is then cancelled this left the table in a grown-but-uninitialized state where subsequent usage via WebAssembly could possibly segfault. Loads from this table are assumed to not be null due to its type, but the runtime implementation was exposed through this cancellation at a preemption point. * Embedders could mutate the store during an epoch callback, such as growing a WebAssembly linear memory. During a bulk `memory.copy` operation, however, the pointers being copied to/from weren't recomputed between preemption points. This meant that if the linear memory moved its base address it could be possible to have a preemption, the embedder manually grows memory, and then on resumption the copy operation uses invalid pointers. * Embedders could execute a GC during epoch callbacks. GC operations such as `array.copy`, like `memory