Support domain specific variable type

[schell]

• adds a type parameter to many types, representing a solver variable type
• removes Variable in favor of tools for writing your own domain specific variable types
• adds some chain/builder-style functions for creating constraints
• moves some other things around where they found better homes
• moves a bunch of trait implementations into a macro
• removes all instances of Arc, Rc and RefCell in favor of OrderedFloat and Option - Solver can now safely be passed between threads with Send and Sync
• generalizes Solver::optimise and removes Solver.artificial as its no longer needed
• bumps the version to 0.4

This provides library users with the ability to specify their own variables. All the structs associated with constructing constraint syntax now take a type variable that represents the domain specific variable type. Variable itself is removed as its trivial to derive it. The tests provide an example. There's a new module derive_syntax for automatically deriving the necessary machinery to write constraints and expressions using the domain specific variable type.

This has two good effects:

1. There's no need for maintaining a HashMap<MyType, Variable> mapping from meaning to cassowary variable.
2. It's now very easy to run n separate solvers - one for each dimension - and not have the types mix. For example:

pub struct Entity {...}
pub enum XVariable {
  Left(Entity), Width(Entity), Right(Entity)
}
derive_syntax_for!(XVariable);

pub enum YVariable {
  Top(Entity), Height(Entity), Bottom(Entity)
}
derive_syntax_for!(YVariable);

type XSolver = Solver<XVariable>;
type YSolver = Solver<YVariable>;

^ Here you can't mix constraints from the two axes without triggering a compile time error, which I think I like. Anyway - there's only a couple tests really, so YMMV.

This is currently a draft PR until I update the documentation.