seal_gas_left should be Weight(u64), not T::Balance (u128)

[atenjin]

seal_gas_left should be Weight(u64), not T::Balance (u128)

Describe the bug
In ink!, code is:

https://github.com/paritytech/ink/blob/master/crates/env/src/api.rs#L93-L100

the gas_left return T::Balance. But the default Balance is u128.

But in substrate pallet-contracts

https://github.com/paritytech/substrate/blob/master/frame/contracts/src/wasm/runtime.rs#L1070

the ctx.ext.gas_meter().gas_left() will return Weight, it's u64.

Thus if call seal_gas_left in ink!, must meet decode error.

[Robbepop]

Good find!
Since Weight is not customizable in the contracts pallet we should change the return type to plain u64.

[HCastano]

@atenjin feel free to open a PR for this!

[cmichi]

Closed via #797.