message: Change parse_string to run validation
diff: #include "parser.hpp"
  #include "leb128.hpp"
  #include "types.hpp"
  +#include "utf8.hpp"
  #include <algorithm>
  namespace fizzy
  @@ -148,7 +149,9 @@ parser_result<std::string> parse_string(const uint8_t* pos, const uint8_t* end)
  if ((pos + size) > end)
  throw parser_error{"Unexpected EOF"};
  - // FIXME: need to validate that string is a valid UTF-8
  + if (!utf8_validate(pos, pos + size))
  + throw parser_error{"Invalid UTF-8"};
  +
  auto ret = std::string(pos, pos + size);
  pos += size;
  