RFR: 8369564: Provide a MemorySegment API to read strings with known lengths [v4]

[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.
> 
> ---
> 
> 
> Benchmark                                 (size)  Mode  Cnt   Score   Error  Units
> ToJavaStringTest.panama_copyLength             5  avgt   30   7.244 ± 0.057  ns/op
> ToJavaStringTest.panama_copyLength            20  avgt   30   7.499 ± 0.140  ns/op
> ToJavaStringTest.panama_copyLength           100  avgt   30  11.997 ± 0.148  ns/op
> ToJavaStringTest.panama_copyLength           200  avgt   30  16.260 ± 0.141  ns/op
> ToJavaStringTest.panama_copyLength           451  avgt   30  26.184 ± 0.184  ns/op
> ToJavaStringTest.panama_readString             5  avgt   30   5.969 ± 0.055  ns/op
> ToJavaStringTest.panama_readString            20  avgt   30   7.750 ± 0.066  ns/op
> ToJavaStringTest.panama_readString           100  avgt   30  14.094 ± 0.181  ns/op
> ToJavaStringTest.panama_readString           200  avgt   30  18.094 ± 0.187  ns/op
> ToJavaStringTest.panama_readString           451  avgt   30  36.052 ± 0.143  ns/op
> ToJavaStringTest.panama_readStringLength       5  avgt   30   4.622 ± 0.076  ns/op
> ToJavaStringTest.panama_readStringLength      20  avgt   30   4.703 ± 0.087  ns/op
> ToJavaStringTest.panama_readStringLength     100  avgt   30   8.117 ± 0.204  ns/op
> ToJavaStringTest.panama_readStringLength     200  avgt   30  10.568 ± 0.231  ns/op
> ToJavaStringTest.panama_readStringLength     451  avgt   30  16.273 ± 0.149  ns/op
> 
> 
> 
> Benchmark                                                  (size)  Mode  Cnt   Score   Error  Units
> FromJavaStringTest.panama_copyStringWithoutNullTerminator       5  avgt   30   4.582 ± 0.028  ns/op
> FromJavaStringTest.panama_copyStringWithoutNullTerminator      20  avgt   30   4.973 ± 0.013  ns/op
> FromJavaStringTest.panama_copyStringWithoutNullTerminator     100  avgt   30   9.229 ± 0.077  ns/op
> FromJavaStringTest.panama_copyStringWithoutNullTerminator     200  avgt  ...

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

  Add benchmarks, and demo setStringWithoutNullTerminator

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/28043/files
  - new: https://git.openjdk.org/jdk/pull/28043/files/43a719eb..53b064f2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28043&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28043&range=02-03

  Stats: 164 lines in 4 files changed: 163 ins; 0 del; 1 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