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

```
StringContext#checkLengths reports bad args count

Since interpolator args are of type Any, it's easy
to write s(args) instead of s(args: _*).  I wonder
if Xlint would have warned me about that.

In tests, interpolation is no longer -Xperimental.