wasm: move implementation details into WasmVm abstraction layer.

[PiotrSikora]

Signed-off-by: Piotr Sikora piotrsikora@google.com

[istio-testing]

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[PiotrSikora]

@jplevyak in the interest of cleaning up the interface, I pulled getFunction() into the abstraction layer. It doesn't leak any VM implementation details, which I think is beneficial. The only downside is that there are no templates for virtual methods in C++, so the interface gets a bit more noisy, but it seems worth it. What do you think?

(I'll migrate registerCallback() and makeGlobal() as well, but I didn't want to spend too much time on this, if you disagree with the direction of this change)

cc @silentdai

[jplevyak]

I am not opposed to this, but it does add some noise. If you like it better, then feel free to merge. If you can wrap up a bit of larger repetitions in macros and you think it is cleaner I would appreciate it. I find that if the macro is defined directly above the repeated use it improves readability and cuts down on toil.

[jplevyak]

Perhaps we could wrap this repetition up in a macro?

[PiotrSikora]

Done.

[PiotrSikora]

@jplevyak I agree that it adds a bit noise. Ideally, we'd be able to get/register any function passed as argument, but that doesn't seem to be possible. I definitely prefer that those methods are part of the runtime implementation, and not leaked into the abstraction layer, though... If you have any better idea on how to achieve that, I'm all ears.