Upstream some Diagrams utilities

In a similar vein to #40, there are various definitions in the `diagrams-lib` library which seem like they'd be useful here. Personally I've ended up vendoring the following because `diagrams-lib` can be a bit of a problematic dependency (it has a big footprint, and doesn't work with Wasm for example: https://github.com/diagrams/diagrams-lib/issues/370):

```hs
type P2 = Point V2
unitX :: (R1 v, Additive v, Num n) => v n
unitX = zero & lensVL _x .~ 1
unit_X :: (R1 v, Additive v, Num n) => v n
unit_X = zero & lensVL _x .~ -1
unitY :: (R2 v, Additive v, Num n) => v n
unitY = zero & lensVL _y .~ 1
unit_Y :: (R2 v, Additive v, Num n) => v n
unit_Y = zero & lensVL _y .~ -1
```

Would you consider copying some of these to this library?

Cc @byorgey

Start by reviewing issue #40 and comparing the listed definitions with the utilities available in diagrams-lib. Identify which utilities are appropriate to copy without adding the problematic dependency; done means the selected utilities are available in this library and covered by the project's existing checks.

Written by the indexing model from the issue text.