Update concat() to accept traversal varargs and remove special treatment of inject child traversals

As proposed on the devlist, updating concat() to accept traversal varags.

Also to update `concat()` to not special treat `inject()` in parameters and use `TraversalUtil.apply` on it as with any other child traversals. 

So the corrected result as inject() behaves by default (The inject() step injects the incoming traverser as the first item in the stream by default) would be:

gremlin> g.inject("a").concat(inject("b"))
==>aa