Deconstructables + simple parsers singletons + emptiness

1. Simple type parsers are exposed via singletons (see i.e. DoubleParser.Instance)
2. New type of parsing strategy - Deconstructables. It is enough for type to have constructor and matching Deconstruct - these members will be used to parse/format object automatically i.e. this structure

readonly struct Address
{
    public string City { get; }
    public int ZipCode { get; }

    public Address(string city, int zipCode)
    {
        City = city;
        ZipCode = zipCode;
    }

    public void Deconstruct(out string city, out int zipCode)
    {
        city = City;
        zipCode = ZipCode;
    }
}

will be automatically formatted to "(City;ZipCode)" format and parsed from same format

3. New concept - emptiness vs null parsing. Now transformers can decide what it meas to them when "" string is parsed as opposed to null string

4. TupleHelper - user can now create complex parser that parse tuple-like structures (records etc.) using common logic

5. Transformables - types can register own transformers using TransformerAttribute. Such custom transformers can benefit from simple dependency injections (currently only TransformerStore object) via constructor

DEPRECATED:
One of next minor releases will be the last one where we support FromText(string) convention. It was only a bridge for consumers bound to targets < .NET Standard 2.1. Now we encourage upgrade to FromText(ReadOnlySpan) - especially since all simple type parsers are exposed via singletons (see i.e. DoubleParser.Instance)