Ranges for foreach over sequence/mapping nodes

[kiith-sa]

Should be the first step to making D:YAML Nodes range based. Also, when this is added, the current opApply() should be deprecated (but kept for one/more releases).

E.g. sequence, mapping, mappingKeys, mappingValues. These will assert that the key is a sequence/mapping.

Example: string[] strings = node.mappingKeys.map!(n => n.as!string).array;

Range primitives of these ranges could be more @safe/pure/nothrow/@nogc than current monolithic opApply().

Future ranges could also include depth/breadth first range over all Nodes and their subnodes. The element of such ranges should include Node packed with current depth and maybe some more info.