Make Context abstract

[frangio]

Other instances where we could do the same if we moved state variable initialization outside of the constructor:

openzeppelin-contracts/contracts/utils/ReentrancyGuard.sol

Lines 22 to 30 in 3f23822

	constructor () internal {
	// Storing an initial non-zero value makes deployment a bit more
	// expensive, but in exchange the refund on every call to nonReentrant
	// will be lower in amount. Since refunds are capped to a percetange of
	// the total transaction's gas, it is best to keep them low in cases
	// like this one, to increase the likelihood of the full refund coming
	// into effect.
	_notEntered = true;
	}

openzeppelin-contracts/contracts/utils/Pausable.sol

Lines 30 to 32 in 3f23822

	constructor () internal {
	_paused = false;
	}

[nventuro]

This is related to ethereum/solidity#8162, abstract should be required.