Include logical property value query operators

[jasongoodwin]

A query should be able to say how multiple property value checks combine:

Eg:
get vertices where name == "john" OR address == "111 elm street"
Also need to be able to bracket and group them: (name = "john" OR address == "111 elm street") AND active = "yes"
Multiple property value checks is easy, but figuring out how to group them in brackets in the proto api would be challenging.
Not sure how to do that.

This is somewhat related to:
#227

[KevinMGranger]

For the proto, you can use a list of queries, and just mark if it's and or or. For example:

// A query for vertices.
message VertexQuery {
    oneof query {
        RangeVertexQuery range = 1;
        SpecificVertexQuery specific = 2;
        PipeVertexQuery pipe = 3;
        PropertyPresenceVertexQuery property_presence = 4;
        PropertyValueVertexQuery property_value = 5;
        PipePropertyPresenceVertexQuery pipe_property_presence = 6;
        PipePropertyValueVertexQuery pipe_property_value = 7;
        MultiVertexQuery and = 8;
        MultiVertexQuery or = 9;
    }
}

// A list of VertexQueries to be used in a
// conjunction (logical and) or a disjunction (logical or).
message MultiVertexQuery {
    repeated VertexQuery queries = 1;
}

(similar for EdgeQuery). This was just a quick PoC-- if separate wrapper types for the rust enums makes more sense, then let's do that.
Edit: right, you do need separate types for impl Into<VertexQuery>. Easy enough to do.