Runtime Implementation

[ec2]

Summary of changes
Changes introduced in this pull request:

• implemented runtime
• Change interfaces for actors to take mutable references
• naive interpreter

Reference issue to close (if applicable)

Closes
#67

Other information and links

There are still a few things that need to be done. Namely, we still need to implement gas calculations. Until we have that, there we obviously cannot calculate the gas cost of calls. Also, we won't be able to verify some of the things that need to be verified. I have marked these as TODO.

Also, I have left the Runtime package in because that is where the Runtime trait lives. It is debatable as to whether we need a Runtime trait and I am heavily leaning on removing it altogether. The initial reason we had the Runtime trait is to create different Runtime for testing, for example. However, we can mock those, which might actually make things a lot more easier. I figured that I'll leave the trait in here meanwhile, and possibly remove it when we start doing the Actors testing... But as an FYI, the VM is currently using the DefaultRuntime struct instead of referencing the trait. It is a LOT more easier to do it like this...

The logic for the Lotus' send functions are in runtime.go and vm.go here https://github.com/filecoin-project/lotus/tree/testnet/3/chain/vm
@austinabell @dutterbutter

[austinabell]

Please write some tests for this logic, seems it isn't doing what you think it is