Support constant for phi

My most-common use case for insect happens to be calculating column widths using `phi` (aka the [Golden ratio](https://en.wikipedia.org/wiki/Golden_ratio)).

It'd be very nice to have it defined as a global-constant, where

```purescript
phi = φ = (1 + sqrt(5)) / 2
```