* 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
