SI-7715 Etapolator s"$_"

Allows `vs map s"How many? $_"`.
    
Or `vs zip vs map s"How many? $_ by $_".tupled`.
    
And `s"How many? $_ by $_ by $_"("one","two","three")`.
    
In a pattern,
    
 ```
    scala> val rx"$_ by ${I(b)}" = "2 by 4"
    b: Int = 4
 ```