RFR: 8259947: (fs) Optimize UnixPath.encode implementation [v4]

[Claes Redestad]

> This patch improves `UnixPath.encode` by reusing `JLA.getBytesNoRepl` (which has fast-paths for common encoding) and avoiding a `toCharArray` call on the input by refactoring the `normalizeNativePath` code to operate on `String`. This might have a cost on files on Mac that need additional native normalization.
> 
> This removes another `ThreadLocal` and a source of `SoftReference`s. Together with the UTF-8 fast-path my UTF-8 encoded file system see substantial speed-ups in a trivial `new File(str).toPath()` microbenchmark.

Claes Redestad has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - Merge branch 'master' into unix_encode
 - Move FileOpen.ToPath micros into top class
 - Move JLA to top, add imports
 - Fold ToPath into FileOpen, add root benchmarks to keep mix comparable
 - Add micro. To properly examine cost of toPath() needs a new File due caching - use FileOpen as a baseline
 - Optimize UnixPath.encode

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2135/files
  - new: https://git.openjdk.java.net/jdk/pull/2135/files/b023c0a5..98c76192

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2135&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2135&range=02-03

  Stats: 3891 lines in 128 files changed: 671 ins; 2864 del; 356 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2135.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2135/head:pull/2135

PR: https://git.openjdk.java.net/jdk/pull/2135