Enhance GenLens to support select chains i.e. _.a.b.c

[cvogt]

hi from @xdotai :)

[julien-truffaut]

@cvogt really nice, thank you.

Could you tell me what error message do you get if an accessor along the chain is not a case class accessor?

[cvogt]

Well something like GenLens[A](_.b.c.d) is simply expanded into GenLens[A](_.b) composeLens GenLens[B](_.c) composeLens GenLens[C](_.d) (assuming .b: B, .c:C). You tell me what Monocle does when you try to use GenLens on a non-case class field :). I didn't check.