RFC: unify directors and backends vcl interface

[Nils Goroll]

backends and directors are both represented as VCL_BACKEND aka struct director,
yet VCL authors need to differentiate between them:

* backends are represented directly as the BACKEND type
* directors are represented as vmod objects, for which a method (.backend() by
  convention only) returns a BACKEND

I don't see why we would need to make this distinction and getting rid of it
would simplify VCL code generation and -refactoring. Consider the case having
backend "b" which you now want to layer under director "d": all instances of "b"
in the vcl now need to be replaced with something like "d.backend()".

I see two obvious options to simplify:

* implement backends as vmod objects

  and/or

* add a default getter method for objects such that

	d == d.backend()

Opinions?