RFR: 8369564: Provide a MemorySegment API to read strings with known lengths [v7]

[Liam Miller-Cushon]

> This PR proposes adding a new overload to `MemorySegment::getString` that takes a known byte length of the content.
> 
> This was previously proposed in https://github.com/openjdk/jdk/pull/20725, but the outcome of [JDK-8333843](https://bugs.openjdk.org/browse/JDK-8333843) was to update `MemorySegment#getString` to suggest
> 
> 
>     byte[] bytes = new byte[length];
>     MemorySegment.copy(segment, JAVA_BYTE, offset, bytes, 0, length);
>     return new String(bytes, charset);
> 
> 
> However this is less efficient than what the implementation of getString does after [JDK-8362893](https://bugs.openjdk.org/browse/JDK-8362893), it now uses `JavaLangAccess::uncheckedNewStringNoRepl` to avoid the copy.
> 
> See also discussion in [this panama-dev@ thread](https://mail.openjdk.org/pipermail/panama-dev/2025-November/021193.html), and mcimadamore's document [Pulling the (foreign) string](https://cr.openjdk.org/~mcimadamore/panama/strings_ffm.html)
> 
> Benchmark results:
> 
> 
> Benchmark                                 (size)  Mode  Cnt    Score   Error  Units
> ToJavaStringTest.jni_readString                5  avgt   30   55.339 ± 0.401  ns/op
> ToJavaStringTest.jni_readString               20  avgt   30   59.887 ± 0.295  ns/op
> ToJavaStringTest.jni_readString              100  avgt   30   84.288 ± 0.419  ns/op
> ToJavaStringTest.jni_readString              200  avgt   30  119.275 ± 0.496  ns/op
> ToJavaStringTest.jni_readString              451  avgt   30  193.106 ± 1.528  ns/op
> ToJavaStringTest.panama_copyLength             5  avgt   30    7.348 ± 0.048  ns/op
> ToJavaStringTest.panama_copyLength            20  avgt   30    7.440 ± 0.125  ns/op
> ToJavaStringTest.panama_copyLength           100  avgt   30   11.766 ± 0.058  ns/op
> ToJavaStringTest.panama_copyLength           200  avgt   30   16.096 ± 0.089  ns/op
> ToJavaStringTest.panama_copyLength           451  avgt   30   25.844 ± 0.054  ns/op
> ToJavaStringTest.panama_readString             5  avgt   30    5.857 ± 0.046  ns/op
> ToJavaStringTest.panama_readString            20  avgt   30    7.750 ± 0.046  ns/op
> ToJavaStringTest.panama_readString           100  avgt   30   14.109 ± 0.187  ns/op
> ToJavaStringTest.panama_readString           200  avgt   30   18.035 ± 0.130  ns/op
> ToJavaStringTest.panama_readString           451  avgt   30   35.896 ± 0.227  ns/op
> ToJavaStringTest.panama_readStringLength       5  avgt   30    4.565 ± 0.038  ns/op
> ToJavaStringTest.panama_readStringLength      20  avgt   30    4.654 ± 0.040  ns/op
> ToJavaStringTest.panama_readString...

Liam Miller-Cushon has updated the pull request incrementally with one additional commit since the last revision:

  Add a dstOffset parameter, stop using StringCharBuffer/CharsetEncoder::encode

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/28043/files
  - new: https://git.openjdk.org/jdk/pull/28043/files/3f6ee815..0593827f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28043&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28043&range=05-06

  Stats: 30 lines in 4 files changed: 3 ins; 14 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/28043.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28043/head:pull/28043

PR: https://git.openjdk.org/jdk/pull/28043