Further performance improvements, and corrections in get_token_string

o An (-'ve valued, typically -1) EOF must never be allowed in
  token_string, as it be converted to 255 -- a legitimate value.
o Comparing against a specific eof() (-1, typically) is more costly than
  detecting +'ve/-'ve.  Since EOF is the only non-positive value allowed
  we can use the simpler test.
o Removed unnecessary test for token_string size, as it is already
  tested in the method, and must never occur in correct code; used an
  assert instead.

test/src/unit-class_parser.cpp

@@ -215,7 +215,7 @@ TEST_CASE("parser class")
                     std::string s = "\"1\"";
                     s[1] = '\0';
                     CHECK_THROWS_AS(json::parse(s.begin(), s.end()), json::parse_error&);
-                    CHECK_THROWS_WITH(json::parse(s.begin(), s.end()), "[json.exception.parse_error.101] parse error at 2: syntax error - invalid string: control character must be escaped; last read: '\"'");
+                    CHECK_THROWS_WITH(json::parse(s.begin(), s.end()), "[json.exception.parse_error.101] parse error at 2: syntax error - invalid string: control character must be escaped; last read: '\"<U+0000>'");
                 }
             }