Use the wasmtime on-demand allocator

[acfoltzer]

For run: this execution mode is one-and-done and not meant to emulate heap limits and such from real C@E, this should use Wasmtime's default allocator. This will both lower the virtual memory footprint of executions and allow unit tests to use more than the C@E heap limit (currently 128MiB)

For serve: StoreLimitsBuilder should let us provide fidelity against the real platform limits