adds a new [de]serialize method for nodes only

[othane]

summary of changes:

• add kowhai_[de]serialize_nodes() to serialise the tree in a flat jason file where the tree path is encoded in a string .. this is more useful for editing by hand in a file, or if you just want to write part of a tree, ie writing every instance of bar in foo, but only bar not the other surrounding elements of foo, ie: element: struct { int untouched1; int bar; int untouched2; } foo[4]; note that this api does not rebuild a kowhai_tree from scratch given the serialised file, instead it merges the data it has into an existing tree with a similar structure (see kowahi_node_not_found_t callback for merging options) .. also it requires (cannot be NULL) as kowahi_get_symbol_t callback to convert a symbol string to a node id and a kowahi_get_symbol_name_t (like the tree api) ... if human readable symbol strings are not used you can just printf and scanf the numbers into the tree, if symbol names are used they must not include '.' or '[' or ']' characters (these are special delimiters in path encoding.
• renamed kowhai[de]serialize() -> kowhai[de]serialize_tree() ... functionality remains unchanged (done just to make clear what type of serialization is being performed, ie not nodes)
• add new error types for new api's
• replace a union type for all kowhai type that was being redefined over and over with a named union called anytype_t
• unit test updated for api name changes and cover all foreseeable error conditions on new kowhai_[de]serialize() api, and of course the successful condition of serializing and de-serializing a kowahi tee's nodes is done

[othane]

pull me !!