Limit gas on contract call

[kroggen]

When a contract calls another one, it should be able to specify a limit of gas that the called contract can use

  contract.call.gas(123)(address, function_name, args)

This feature appears to be partially implemented. It can be reviewed to check for viability.

[kroggen]

Probably the above is not required, if #239 is implemented it can be done like this:

local gas_before = contract.gasLeft()
contract.call(address, function_name, args)
local used_gas = gas_before - contract.gasLeft()
assert(used_gas <= 12500, "contract call used too much gas: " .. tostring(used_gas))