Add custom resource limiting for instances

[aturon]

Lucet currently provides configurable limits for things like linear memory growth, but these limits are relatively "static" in that they are set at a specific value when setting up an instance.

For some use-cases, resource limits need to vary based on other conditions on the host, e.g. when attempting to limit both the WebAssembly heap and the allocations resulting from calls into the host.

This PR addresses these additional use-cases by adding an optional, more dynamic ResourceLimiter trait that provides hooks for "approving" requests for additional resources dynamically, as they occur. A resource limiter can be configured as part of building an instance.

For now, the only check provided by ResourceLimiter is memory_growing, invoked during Instance::grow_memory.

[aturon]

I haven't dug into testing in Lucet yet, but if this PR generally seems acceptable I'm happy to add a test as well.

[aturon]

Related draft PR for wasmtime.