Add support for Lua smart contracts

[garious]

• One Lua stack per contract
• Use coroutines to move forward across multiple events

here's the AST for a smart contract that waits for either a timestamp or an abort signature:

Or((Timestamp(some_date), Payment(tokens, to)), (Signature(from), Payment(tokens, from)))

In Lua, that might look like this:

    while true do
       local event = bank:wait_for_event()
       if isinstance(event, Signature) and event.signature == from then
          bank:transfer(tokens, from)
       elseif isinstance(event, Timestamp) and event.date > some_date then
          bank:transfer(tokens, "0ddba11")
       end
    end

[garious]

Depends on #159

[garious]

Might end up more like this where accounts are passed in and state is passed through account userdata:

       local event = deserialize(data)
       if isinstance(event, Signature) then
          accounts[1].tokens = accounts[1].tokens + tokens
       elseif isinstance(event, Timestamp) and event.date > some_date then
          accounts[2].tokens = accounts[2].tokens + tokens
       end

[garious]

Withdrawing this one. We'll focus on performance and concurrency.