RFR: 8341566: Adding factory for non-synchronized CharSequence Reader

[Markus KARG]

This Pull Requests proposes an implementation for [JDK-8341566](https://bugs.openjdk.org/browse/JDK-8341566): Adding the new method `public static Reader Reader.of(CharSequence)` will return an anonymous, non-synchronized implementation of a `Reader` for each kind of `CharSequence` implementation. It is optimized for `String`, `StringBuilder`, `StringBuffer` and `CharBuffer`.

In addition, this Pull Request proposes to replace the implementation of `StringReader` to become a simple synchronized wrapper around `Reader.of(CharSequence)` for the case of `String` sources. To ensure correctness, this PR...
* ...simply moved the **original code** of `StringBuilder` to become the de-facto implementation of `Reader.of()`, then stripped synchronized from it on the left hand, but kept just a synchronized wrapper on the right hand. Then added a `switch` for optimizations within the original code, at the exact location where previously just an optimization for `String` lived in.
* ...added tests for all methods (`Of.java`), and applied that test upon the modified `StringBuilder`.

Wherever new JavaDocs were added, existing phrases from other code locations have been copied and adapted, to best match the same wording.

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Commit messages:
 - StringReader delegates to Reader.of(String)
 - Reader.of(String)

Changes: https://git.openjdk.org/jdk/pull/21371/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21371&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341566
  Stats: 458 lines in 3 files changed: 411 ins; 36 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/21371.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21371/head:pull/21371

PR: https://git.openjdk.org/jdk/pull/21371