RFR: 8291641: Optimize StackTraceElement.toString()

[David Schlosnagle]

I would like to contribute an optimized version of `StackTraceElement#toString()` that uses a single StringBuilder throughout creation to avoid intermediate `String` allocations. `StackTraceElement#toString()` is used in a number of JDK code paths including `Throwable#printStackTrace()`, as well as many JDK consumers may transform `StackTraceElement` `toString()` in logging frameworks capturing throwables and exceptions, and diagnostics performing dumps.

Given this usage and some observed JFR profiles from production services, I'd like to reduce the intermediate allocations to reduce CPU pressure in these circumstances. I have added a couple benchmarks for a sample `Throwable#printStackTrace()` converted to String via `StringWriter` and individual `StackTraceElement` `toString`. The former shows ~15% improvement, while the latter shows ~40% improvement.

Before

Benchmark                               Mode  Cnt       Score      Error  Units
StackTraceElementBench.printStackTrace  avgt   15  167147.066 ± 4260.521  ns/op
StackTraceElementBench.toString         avgt   15     132.781 ±    2.095  ns/op


After

Benchmark                               Mode  Cnt       Score      Error  Units
StackTraceElementBench.printStackTrace  avgt   15  142909.133 ± 2290.720  ns/op
StackTraceElementBench.toString         avgt   15      78.939 ±    0.469  ns/op

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

Commit messages:
 - Precompute StackTraceElement toString length
 - Merge remote-tracking branch 'origin/master' into ds/StackTraceElement
 - Optimize StackTraceElement.toString()
 - Add StackTraceElementBench

Changes: https://git.openjdk.org/jdk/pull/9665/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9665&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291641
  Stats: 154 lines in 2 files changed: 139 ins; 5 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/9665.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9665/head:pull/9665

PR: https://git.openjdk.org/jdk/pull/9665