Better support for string comparison patterns

[Daejun Park]

when a symbolic string is compared with different-sized strings, fixing the size of symbolic strings is not convenient, e.g.,:

function check_foo(string memory x) {
    if (keccak256(abi.encodePacked(x)) == keccak256(abi.encodePacked("ab"))) ...
    else if (keccak256(abi.encodePacked(x)) == keccak256(abi.encodePacked("cdef"))) ...

special handling of string type symbols or support for symbolic sized arrays is desired.