Optimize YamlScalarNode.Load and YamlMappingNode.Load

[lahma]

Improving scalar loading performance a bit as it's done a lot for common files. Check for "null" marker faster and don't do try-catch in a loop. I fixed the saltern example YAML file for easier testing.

The OrderedDictionary change looks large, but it seems that it has been stored in Git with wrong newlines and always pops up as modified otherwise. If you check ignore whitespace in PR view that should help.

YamlDotNet.Benchmark.YamlStreamBenchmark

Diff	Method	Toolchain	Mean	Error
Old	LoadLarge	Default	61.27 ms	0.538 ms
New		Default	55.86 ms (-9%)	0.891 ms