Rust: add support for create_vm and execute

[axic]

Part of #168.

[axic]

Some notes:

We've got evmc_instance which must be returned by create; what the ffi really needs is that the pointer returned has the members where evmc_instance expects it, but what most VMs do is they store their private instance details after the evmc_instance in the same continuous memory area.

evmc_instance has basically one major function pointer: execute, which also receives evmc_instance as a parameter as a way to access its context

There is code do to the above, but what's missing is the user extending it with private data. I suppose that must be done with macros. The macro takes a struct type and creates the wrapper struct around it doing all the above

[axic]

Should have here a large comment explaining this will lay out memory in a format acceptable to C (repr(C)) and that's why we can include both data structures here.

[jakelang]

will do

[axic]

Please separate the VM name and the name of the VM struct.

[axic]

I mean the VM name may contain space and this will fail in that case.

[jakelang]

discussed offline. figuring this out later

[axic]

Should wrap this in std::catch_panic.

[axic]

This is passing vmtester now with the macro monstrosity. At least locally.

[axic]

@jakelang @chfast how about we split out InterfaceManager from this PR and rename it to ExecutionContext? It should also have tests on its own.

Then we can finish the core wrapper with more confidence.

[axic]

Superseded by #262.