Add feature so that VM can request inserting code into the state

[axic]

For example Hera could request to insert precompiles at certain addresses.

I would imagine this could work as adding a function create_account(address, code, balance) which can be called within execute. We trust the VM to do anything already, so this shouldn't raise any permission questions.

[chfast]

I don't mind to have it in EVMC, but there might not be so easy to implement it on Client side because they usually does not expose API for it directly.

[axic]

I was thinking about this and of course it has to be optional, but instead of in execute, I think it should be in some kind of initialisation stage. Actually that makes sense with #44. I am not fully sure about it, probably better to skip this for ABIv5.

[chfast]

I will definitely include this in the design of 2-phase initialization.

I'd like to put this in a "experimental" namespace to indicate this is not a stable API and implementation is not mandatory. Then we could include this as it is.