RFR: 8353795: Add Writer.of(StringBuilder) [v2]

[Markus KARG]

> This Pull Requests proposes an implementation for [JDK-8353795](https://bugs.openjdk.org/browse/JDK-8353795): Adding the new method `public static Writer Writer.of(StringBuilder)`, providing a non-synchronized Writer implementation optimized for writing into `StringBuilder`.
> 
> A basic test is provided to proof that the new `Writer` behaves as expected. For comparison, the same test is also run against `StringWriter`.

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

 - Undone copyright update of otherwise unchanged file.
 - Update Of.java
   
   Applied changnes proposed by @liach: "the default toString already includes id=..., so I usually don't provide an explicit override to make the code concise."

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Changes:
  - all: https://git.openjdk.org/jdk/pull/24469/files
  - new: https://git.openjdk.org/jdk/pull/24469/files/75efb7fd..ea435ed1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24469&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24469&range=00-01

  Stats: 7 lines in 2 files changed: 0 ins; 5 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/24469.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24469/head:pull/24469

PR: https://git.openjdk.org/jdk/pull/24469