add contract.gasLeft()

[kroggen]

This PR adds the contract.gasLeft() function to retrieve the remaining gas in the contract execution

Closes #239

There is a problem though:

The lua_pushinteger() function accepts a value of type lua_Integer, that is signed int64, and the lua_gasget() function returns an unsigned int64

So if the value of gas is too big, it may appear for the Lua code as a negative value (to be tested)

[kroggen]

One solution is to use bignum for gasLeft()

But then other gas related functions should use bignum as well, for consistency

system.getAmount() and contract.balance() do not use bignum 😕

[kroggen]

When the gaslimit is not set on the transaction, or when it is set to unlimited, the gasLeft is set to UINT64_MAX

So the gaslimit may not be so useful to detect if the txn is about to run out of gas due to the txn creator's balance being low

It may be useful only to limit gas usage on a called contract. But the method used for that would make the transaction to fail (and the changes be reverted). It would be useful to have a way to set a limit and do not revert the txn, like a protected call that just returns false if it runs out of gas

[kroggen]

Maybe this PR do not need to be applied on V4, and can be left to the next hardfork

[hayarobi]

In the current version, gas usage is increased on garbage collection. This means that it is virtually impossible for the user to determine the actual remaining gas since it is executed after the contract function call is returned. We need to change the way gas is calculated first, for this function to be meaningful.

[kroggen]

This is changed on V4, this commit disables gas consumption for garbage collection

Although not tested

[kroggen]

But the "remaining gas" is computed based on the transaction "gas limit", and if it is not specified, it is set to UINT64_MAX, even if it is higher than what the account's balance can pay.

The reason may be because the account can receive tokens while the transaction is executed, on one of the called contracts, so the execution is not limited on the initial balance of the account

It turns out that the number is huge and makes no much sense when used on contract.gasLeft()

Maybe this can be implemented later when the gas limit per block is implemented