Support Task return value serialization

[blairconrad]

Split from #79, as proposed by @CableZa.

[blairconrad]

@CableZa and friends, I have a proof-of-concept of serializing a Lazy<int> (because it was easy and available on all platforms) that I think I can expand to support Task and pretty much anything we want, for all serializers. I'm going to assign this issue to myself.

[blairconrad]

Sketch of my plan: to implement type converters that will transform an unserializable type into a serializable representation, before we hit the serializer, and likewise to convert back.
Rough and provisional interface for such a converter:

interface ITypeConverter
{
     /// <summary>
    /// Potentially converts an unserializable object to a more serializable form.
    /// </summary>
    /// <param name="input">An input object.</param>
    /// <param name="output">An output object. Will be assigned to a simpler representation of <paramref name="input"/>, if this converter knows how.</param>
    /// <returns><c>true</c> if the conversion happened, otherwise <c>false</c>. Good for building a chain of responsibility.</returns>
    bool ConvertForSerialization(object? input, out object? output);

    /// <summary>
    /// Potentially converts the serializable form of an object back to its unserializable form.
    /// </summary>
    /// <param name="deserializedType">The desired deserialized type.</param>
    /// <param name="input">An input object.</param>
    /// <param name="output">An output object. Will be reconstituted from it's simpler representation as <paramref name="input"/>, if this converter knows how.</param>
    /// <returns><c>true</c> if the conversion happened, otherwise <c>false</c>. Good for building a chain of responsibility.</returns>
    bool ConvertForDeserialization(Type deserializedType, object? input, out object? output);
}

That'll probably change a bit as I work. My initial plan is to build in a few converters for some framework types, and allow a plugin mechanism for clients to add their own.