allow Sequential to be used in "using"* dependencies

[mpenet]

This allows to skip a vec call when building the dependency list dynamically, which imho is a bit more user friendly.

[stuartsierra]

The canonical form of using dependencies is a map. The vector representation is just syntactic convenience.

Programmatically-generated dependencies should be maps, not sequential collections.

[mpenet]

I find this quite odd since the API specifies maps or vectors (canonical form or not) , and usually unless you require some of the performance/access guarantees of vectors you should allow any Sequential, I have yet to think of a single c.core functions that does this for instance (unless it's a vector op only). Additionally It costs you nothing to allow it...