Gremlin: Handle E() and addE() steps

[chungquantin]

Is your feature request related to a problem? Please describe.

• E(): The E()-step is meant to read edges from the graph and is usually used to start a GraphTraversal. Unlike the related V() it cannot be used mid-traversal.
• AddE(): Reasoning is the process of making explicit what is implicit in the data. What is explicit in a graph are the objects of the graph — i.e. vertices and edges. What is implicit in the graph is the traversal. In other words, traversals expose meaning where the meaning is determined by the traversal definition. For example, take the concept of a "co-developer." Two people are co-developers if they have worked on the same project together. This concept can be represented as a traversal and thus, the concept of "co-developers" can be derived. Moreover, what was once implicit can be made explicit via the addE()-step (map/sideEffect).

Describe the solution you'd like
Add step handler in process/exec.rs and collect method for edges in process/collector.rs

Additional context
Handle token terminator and source step streaming happens with property step