Introduce sol_structname option

[siburu]

As an extension of MessageOptions.

Input (.proto)

package very.long.package.name;

message Foo {
    string x = 1;
    int64 y = 2;
}

message Bar {
    option (.solidity.sol_structname) = "Bar";
    string x = 1;
    int64 y = 2;
}

Output (.sol)

library VeryLongPackageNameFoo {
    struct Data {
        string x;
        int64 y;
    }

    ...
}

library Bar {
    struct Data {
        string x;
        int64 y;
    }

    ...
}