* moved load procs to yaml/loading.nim
* moved input YamlStream into ConstructionContext
* made ConstructionContext a non-ref object
* harmonized code style & comments
* renamed yaml/serialization.nim to yaml/native.nim
* removed default value objects,
use Nim2 default vaules for fields instead.
* moved dump procs to yaml/dumping and introduced Dumper
* modified serialization procs to use ctx as first argument,
so that prefix calls can be used
* changed defaults for presentation to be more like other
YAML implementations, e.g. no directives by default.
* Removed deprecated procs in DOM API
* Removed PresentationStyle – use the setXStyle procs of Dumper
instead.
* Fixed some warnings emerging from transitioning to Nim2
* added loadFlattened. Fixes#117
* fixed an error where recursive nodes were not loaded
properly into YamlNodes.
* made dump with asNone raise an error when a cycle is
encountered, instead of running into an endless loop.
Sorry that this messes with the style here, but it would be nice to have this syntax removed given how long it's been deprecated (https://github.com/nim-lang/Nim/pull/20199).
