A mini-DSL for Measures.

For an example, try this program:

```
import           Diagrams.Backend.SVG.CmdLine
import           Diagrams.Core.Types
import           Diagrams.Prelude

main = defaultMain (square 1 # lw (Normalized 0.3 `atLeast` Output 2) # frame 0.5)
```

Then try choosing various output sizes.  The line width scales with the output size, until as the output size gets smaller, at some point the line width hits a minimum width of 2 pixels and doesn't go below that no matter how small the output gets.