Add type safe projection

[To-om]

This PR adds type safe version of project. This uses a ProjectionBuilder class to build HList type. I choose to not expose projection label (only order of projections counts):

graph
        .V()
        .out("created")
        .project(_
          .apply(_.value(Key[String]("name")))
          .and(_.in("created").count()))
        .head()
// returns "lop" :: 3 :: HNil with correct type

[mpollmeier]

This is great, thank you! I'd prefer to not expose HList to the user, instead we should transform it to tuples, which is possible (used e.g. in select). What do you think?

Also note that #241 by @jeremysears is going in a similar direction - I tried to chip in but didn't manage to finish it.

[To-om]

I agree with you. HList complexity should not be exposed to user.

[mpollmeier]

I just took the liberty to refactor the testcase, let me know if you like it: a141bb4

For the 'simple' case of having only one By modulation, the result type is always derived as a Tuple1, which is clunky to work with - have a look at the test I added. I'm guessing shapeless can map that to the wrapped type, but no idea how clunky that becomes... thoughts?

This is fun!

[To-om]

I don't think that Tuple1 is problematic as the use of project for only one projection is non-sense (you can simply remove the project step).
If you really want to unwrap Tuple1, it is possible to add a new layer of implicit that transforms Tuple1[T] into T and other types by themselves. IMHO it adds more useless complexity.

[mpollmeier]

🤦‍♂️ of course, you're right.

I dropped that nonsense test, moved ProjectionBuilder to it's own file: #273
I'll merged to master once CI is green, then it will release automagically. Thank you!