compile time constant variable declaration should allow use of pure functions

[thegostep]

Description

compile time constant variable declaration should allow use of pure functions

Environment

• Compiler version: 0.6

Steps to Reproduce

// current
uint256 public constant ATTRIBUTE_ID = uint256(keccak256(bytes("attribute")));

// expected
uint256 public constant ATTRIBUTE_ID = stringToUint("attribute");

function stringToUint(string memory attribute) public pure returns (uint256 attributeID) {
	return uint256(keccak256(bytes(attribute)));
}

[chriseth]

Duplicate of #3122
The reason we did not implement this yet is mainly because the keyword pure does not really mean the same thing as "compile-time constant" - it just means "does not read blockchain state".