RFR: 8265237: String.join and StringJoiner can be improved further [v4]

[Peter Levart]

> While JDK-8148937 improved StringJoiner class by replacing internal use of getChars that copies out characters from String elements into a char[] array with StringBuilder which is somehow more optimal, the improvement was marginal in speed (0% ... 10%) and mainly for smaller strings, while GC was reduced by about 50% in average per operation.
> Initial attempt to tackle that issue was more involved, but was later discarded because it was apparently using too much internal String details in code that lives outside String and outside java.lang package.
> But there is another way to package such "intimate" code - we can put it into String itself and just call it from StringJoiner.
> This PR is an attempt at doing just that. It introduces new package-private method in `java.lang.String` which is then used from both pubic static `String.join` methods as well as from `java.util.StringJoiner` (via SharedSecrets). The improvements can be seen by running the following JMH benchmark:
> 
> https://gist.github.com/plevart/86ac7fc6d4541dbc08256cde544019ce
> 
> The comparative results are here:
> 
> https://jmh.morethan.io/?gist=7eb421cf7982456a2962269137f71c15
> 
> The jmh-result.json files are here:
> 
> https://gist.github.com/plevart/7eb421cf7982456a2962269137f71c15
> 
> Improvement in speed ranges from 8% (for small strings) to 200% (for long strings), while creation of garbage has been further reduced to an almost garbage-free operation.
> 
> So WDYT?

Peter Levart has updated the pull request incrementally with one additional commit since the last revision:

  Fork JVM 3 times in JMH test to smooth out variance in a particular fork

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3501/files
  - new: https://git.openjdk.java.net/jdk/pull/3501/files/a2d5e819..84af9e6c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3501&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3501&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3501.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3501/head:pull/3501

PR: https://git.openjdk.java.net/jdk/pull/3501