Add jsonpointer attribute to each OpenAPI node

[mykulyak]

In many places we need to print some useful information about an OpenAPI nodes, having only a reference to a node. For example, in assertions there is often a need to point the problematic place.

We can alleviate this need in two ways:

• by attaching line/col/pos information from the source file to the message
• by assigning each node a jsonpointer