Enable calls to external library functions

[charles-liu]

library D 
{ 
    function double(uint self) public returns (uint) { 
        // do something with side effects
        return 2*self;
    }
}

contract C {
    using D for uint;
    function f(uint a) public returns (uint) {
        return a.double();
    }
}
// ----
// Warning 2018: (12-79): Function state mutability can be restricted to pure

This is a test case from Solidity. We assume that double function will do something with side effects to modify the storage of C, so it’s not declared as internal pure.
It is not possible both in Solidity++ 0.4 or 0.8, because the DELEGATECALL instruction is not implemented on Vite VM and some functionalities of libraries are disabled in the compiler.