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

[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?

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Commit messages:
 - Make JavaLangAccess JLA field private static final
 - Alternative String.join

Changes: https://git.openjdk.java.net/jdk/pull/3501/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3501&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265237
  Stats: 97 lines in 4 files changed: 61 ins; 18 del; 18 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