RFR: 8341566: Add Reader.of(CharSequence) [v7]

[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.

Markus KARG has updated the pull request incrementally with two additional commits since the last revision:

 - assertThrows instead of expectedExceptions
 - ordered jtreg tags according recommendation in https://openjdk.org/jtreg/tag-spec.html#ORDER

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21371/files
  - new: https://git.openjdk.org/jdk/pull/21371/files/75cb4fc6..c0729baf

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21371&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21371&range=05-06

  Stats: 16 lines in 1 file changed: 1 ins; 1 del; 14 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