Clean up toString implementation

[travisbrown]

This is a follow-up to #7. I've now got toString for Expr working well enough that it can be used to round-trip the unnormalised prelude, which was my goal for 0.1.0, but the implementation is still a disaster. I'd originally been using it for debugging and didn't really care about producing valid Dhall code, and I just threw together the current version in the past couple of days. It parenthesises unnecessarily, probably still gets precedence wrong in some cases, is an unmaintainable mess, etc.

We'll also probably want some kind of Dhall code pretty-printing at some point, but I'll open a separate issue for that.

[travisbrown]

One specific thing to note is that we do have a property test in ToStringSuite that is supposed to validate that we can take arbitrary (not-necessarily-well-typed) expressions, print them with toString, and parse them to get the original value.

This test is ignored right now because it fails sometimes, but that could be an issue with the Arbitrary instance, and our Shrink isn't good enough to make the failure messages useful.