Improve printing of strings

[som-snytt]

Ports scala/scala#10897

Why was the ticket worth tackling?

It's a port of a Scala 2 support ticket, which means it is a performance improvement worth paying for. The code is not shared with Scala 2 but is similar in principle.

How I fixed it

The bottleneck is string.flatMap(escapedChar). At the urging of the reviewer, reduced code duplication between PlainPrinter and SourceCode by sticking it in util.Chars.

Why is this PR worth reviewing?

It would be more compelling with a jmh demonstration, aside from correctness. This PR is like grading homework, thankless but instructional.

[Mikhail42]

It would be nice to add at least a few tests. I've not found them here (https://github.com/scala/scala3/tree/main/compiler/test/dotty/tools/dotc/printing)

[som-snytt]

I would do something about the code duplication before undrafting.

Everything breaks if strings are broken, and on scala 2 it was also a zinc dependency. A useful test would be a scalacheck, as there are many boundary conditions. An existing test broke because it noticed whether Unicode escapes were lowercase. (I did not argue with the test.)

[noti0na1]

Sorry for so long to back to this PR. Other than the code duplication, the changes look good to me.

[noti0na1]

Can we move the functions to compiler/src/dotty/tools/dotc/util/Chars.scala to avoid duplication?

[som-snytt]

I guess we can!

[noti0na1]

We don't need the parameter b here, since it is not changed.

[som-snytt]

Inlined quadNibble just for the pleasure of watching it unroll. It would be nice to invest in a jmh test to verify that it doesn't have unintended consequences. I misplaced the test for Scala 2 that demonstrated it.