Better node display

[benwr]

Change the Display instance for Node. Previously it printed a simple pre-order traversal, which was sometimes ambiguous because e.g. for nested n-ary functions it wasn't clear which things were arguments.

Now, this should produce a visual that is (a) unique, and (b) can be parsed back into an equivalent Node.

[ISibboI]

Thank you a lot, this looks great! Can you add some tests with some simple and more complicated expressions whose Display output parses into the same operator tree?

[benwr]

How would you feel about adding a dev dependency on quickcheck? I might want to randomly generate operator trees and check that they round-trip to the same trees.

[ISibboI]

I am totally fine with new dev dependencies.

[ISibboI]

It seems like propagating the precision does not work. We can either just not support floating point precision, or we fix that somehow.