VM: Implement indexer fallback for missing properties.

[schungx]

Right now, the VM does not fallback to trying an indexer (getter or setter) when access to a property failed.

This PR implements this behavior so it'll match the AST walker.

[ImTheSquid]

Unused variable warnings should be handled through this kinda deal:

fn foo(
#[cfg_attr(feature="no_*", allow(unused))] var: Typ
) {}

Other than that LGTM

[schungx]

Unused variable warnings should be handled through this kinda deal:

Are there unused? Let me double check... I thought I caught them all...

EDIT: Ah, gotcha.

[schungx]

Unused variable warnings should be handled through this kinda deal:

It was too many attributes to put on the parameters, I ended up using this:

let _ = (target, key, value, fail_silently, pos);

This will get removed by the compiler during optimization so it won't affect performance.

I see a couple of places in grain that this style is used, so I suppose this is OK! 😆