Escape control characters in keys

Previously a key like:

        "a\u0000\u0001b" = 1

Would get written with literal control characters, rather than escapes:

        "a<00><01>b" = 1

The "valid/key/quoted-unicode" test from toml-test would fail with this,
although it seems they're not run automatically(?)

Can also reproduce with something like:

        % cat test.cpp
        #include <toml.hpp>
        #include <iostream>

        int main()
        {
                const auto data  = toml::parse("test.toml");
                std::cout << data << "\n";
                return 0;
        }

        % cat test.toml
        "a\u0000\u0001b" = "a\u0000\u0001b"

        % c++ -I. test.cpp

        % ./a.out
        "ab" = "a\u0000\u0001b"

        % ./a.out | hexdump -C
        00000000  22 61 00 01 62 22 20 3d  20 22 61 5c 75 30 30 30  |"a..b" = "a\u000|
        00000010  30 5c 75 30 30 30 31 62  22 0a 0a                 |0\u0001b"..|

toml/serializer.hpp

@@ -63,7 +63,19 @@ format_key(const std::basic_string<charT, traits, Alloc>& k)
             case '\f': {serialized += "\\f";  break;}
             case '\n': {serialized += "\\n";  break;}
             case '\r': {serialized += "\\r";  break;}
-            default  : {serialized += c;      break;}
+            default: {
+                if (c >= 0x00 && c < 0x20)
+                {
+                    std::array<char, 7> buf;
+                    std::snprintf(buf.data(), buf.size(), "\\u00%02x", static_cast<int>(c));
+                    serialized += buf.data();
+                }
+                else
+                {
+                    serialized += c;
+                }
+                break;
+            }
         }
     }
     serialized += "\"";