Correctly decode non-utf8 strings from slither printer

[ggrieco-tob]

Tests are missing, but this should work.

[Boyan-MILANOV]

Tested locally and it works. Echidna successfully solves the following test:

contract CoverageString {
    bytes16 key = "\xf8";
    function test(string memory input) public {
       bytes memory x = bytes(input);
       if (x.length > 0)
         assert(key[0] != x[0]);
    }
}