near-vm: Pick a consistent location to store instance/artifact data

[nagisa]

Today when loading/instantiating an Artifct, we store some information with the Engine (i.e. maps with the machine code, VMSharedSignatures, etc.) and some of the information is contained within the Artifact itself.

This makes consistent management of lifetime difficult – dropping the Artifact will drop a lot of information about a wasm module, but not the data allocated into the Engine (as far as I know.)

We should pick a consistent strategy and either allocate everything into an Engine (this requires implementing custom lifetime management, but allows us to ultra-optimize the performance characteristics by utilizing e.g. arena allocators), or keeping everything with the Artifact instances (straightforward data lifetime management.)

This in part also extends to Instances. Today Instance internally contains data about values of globals and similar runtime structures, while referencing the information from the Engine for e.g. machine code.

Worth noting that storing stuff into Engine (or a Store in the parlance of the WebAssembly specification) is what the WebAssembly specification calls for. It would be necessary to make things like webassembly module linking proposal work at all.