Backwards-compat mode for gremlin/tinkerpop

[linas]

Create a backwards-compat mode for gremlin/tinkerpop. Or at least steal some ideas from there.

[linas]

This includes issue #1855 which is called "the property graph model" in tinkerpop.

[linas]

This includes #1893 which calls for additional virtual links to simplify the syntax of patterns.

[linas]

Some of what is needed here is demoed in value-of.scm

There are also some nice programming styles that can be stolen from https://grakn.ai