Printable should adopt a `f"`-like handling of `%`

Currently to do format Strings with the printable String, you have to use verbose method invocations, eg.
```scala
printf(p"$myUInt == ${Hexadecimal(myUInt)}\n")
```
The default is decimal so no need there, but to get hex you have to use `Hexadecimal`. This causes people to use C-style format Strings:
```scala
printf(p"%d == %x\n", myUInt, myUInt)
```

In Scala, you can do format strings with `f"` like so:

```scala
println(f"$myInt == $myInt%x")
```
We should do something similar for Printable

**Problem**
Because `%` wasn't needed by printable, I stupidly didn't require it to be escaped, so changing this behavior for `p"` would be a backwards incompatible change.

So, the answer is that we should probably introduce a new interpolator. We could choose a different single-character, or perhaps do `pf"..."` for "printable format", eg.
```scala
printf(pf"$myUInt == $myUInt%x\n")
```

**Type of issue**: feature request

**Impact**: API addition (no impact on existing code) | API modification

**Development Phase**: proposal

**What is the use case for changing the behavior?**

See above