8377939: Improving efficiency of StreamEncoder::append(CharSequence) and StreamEncoder::(CharSequence, int, int)

[Markus KARG]

Core-Lib Devs,

as proposed in https://bugs.openjdk.org/browse/JDK-8377939 I would like 
to improve efficiency of StreamEncoder::append(CharSequence) (and in 
turn OutputStreamWriter::append(CharSequence)) for non-String and 
non-CharBuffer values, which seems to be safe and simple:

* No API or behavior is changed.
* StreamEncoder is final and is only used internally of OpenJDK, and is 
not leaked.
* AFAIK the proposed change has no side effects other than being 
potentially more efficient.
* Only few classes make actual use of StreamEncoder; OutputStreamWriter 
is probably the most popular one, and immediately provides potential 
benefit to I/O-bound applications dealing with StringBuilder-crafted output.

I have meanwhile coded the change (see diff at 
https://github.com/openjdk/jdk/compare/master...mkarg:jdk:StreamEncoder-uses-CharsSequence-getChars) 
and tested it successfully using OpenJDK Github Actions (see result at 
https://github.com/mkarg/jdk/actions/runs/22259626078).

Are you fine with me opening a Pull Request on Github?

Thanks!

-Markus