Support short-hand for creating codecs/converters

[matanlurey]

Lots of Dart and Flutter users don't even know about package:convert or dart:convert, or when they do it seems difficult to get started - most code samples I've seen folks write static functions in the form of T convert<S, T>(S input) and ignore these packages.

Could we consider adding some simple short-hand classes/functions to make use easier?

• [ ] A MapConverter that uses a backing Map<S, T> to do simple conversions:

class MapConverter<S, T> implements Converter<S, T> {
  const MapCodec(Map<S, T> map) => ...
}

final hexToName = const MapConverter(const {
  0x00: 'Zero',
  0x01: 'One',
  0x02: 'Two',
});

main() {
  print(hexToName.convert(0x01)); // Outputs 'One'.
}

• [ ] A top-level asConverter<S, T>(S Func1<T> function)

main() {
  var /*Converter<int, String>*/ hexToName = asConverter((int value) {
    switch (value) {
      case 0x00: return 'Zero';
      ...
    }
  });
}

• [ ] A top-level asCodec<S, T>(Converter<S, T> encoder, Converter<T, S> decoder)

main() {
  var /*Codec<int, String>*/ hexCodec = asCodec(hexToName, nameToHex);
}

[nex3]

I'd love to see asConverter() and asCodec() as constructors on the Converter and Codec classes. Constructors are the easiest thing to add to core libraries, too, since they don't have any backwards-incompatibility edge cases.

I'm down with MapConverter if you think that use-case is widespread enough to be worth a class.