Add Serdes.Default

[bartelink]

Also refactor, pushing the BytesEncoder logic out to 'add' NewtonsoftJson.Serdes.SerializeToUtf8 as advertised in #94

[deviousasti]

This is the only member accessible in Options anyway - we can let static ctor do lazy loading.

    static let def = lazy Options.Create()

    /// <summary>Analogous to <c>JsonSerializerOptions.Default</c> - allows for sharing/caching of the default profile as defined by <c>Options.Create()</c></summary>
    static member Default : JsonSerializerOptions = def.Value

to

    /// <summary>Analogous to <c>JsonSerializerOptions.Default</c> - allows for sharing/caching of the default profile as defined by <c>Options.Create()</c></summary>
    static member val Default = Options.Create()