Path-like generic get.

cjson_get(node, segments) will lookup a child node by traversing into arrays
and objects. Segments is a null seperated list of path segments and should end
in null ("a\0b\0c\0").

The addition of the cjson_jestr_normalize simplifies array and object lookups
by normalizing the input to the standard jestr representation. It will resolve
oddities like array indices input as unicode escapes.