* Add framework to support more optional types
This PR add a framework to rpc server by using more templates than macros to handle optional types.
Now rpc server recognize both `std/options` and `results.Opt` as optional type for the parameters.
If needed user can add more optional types by overloading `rpc_isOptional` template.
Now aliases to optional types also works.
* Don't expose types used internally by the wrapper
* Improve resilience against null fields
* Fix client processMessage when handling error
* Improve both client and server resilience against fields and elements with null value
* Upgrade rpc router internals
* use new chronos asyncraises
* Fix style mismatch
* Fix nim v2 compilation error
* Addresing review
* Remove unnecessary custom serializer and let the library do the work
* fix error message
* Update readme.md
