update for linear

Accompanies https://github.com/diagrams/diagrams-lib/pull/212

This is nearly done; it remains to decide what to do about `force-layout`, and the minor name collision of `size`.  I think the choices with `force-layout` are port it to linear (it has [no revdeps but `diagrams-contrib`](http://packdeps.haskellers.com/reverse/force-layout)) or create `vector-space` class instances for `V2` and other types, in `diagrams-contrib`.
I've got rid of unnecessary `Backend` constraints in `-lib`, is there any need for them? Also there's a new `RealFloat n, Data n => DataFloat n` in `-core` since it's so common.