RFR: 8316660: (fs) Files.lines implementation can use MemorSegment for file mapping [v2]

[Per Minborg]

> This PR proposes using a `MemorySegment` instead of a `ByteBuffer` in the `Files::lines` implementation (in the internal class `FileChannelLinesSpliterator`).
> 
> The old solution deterministically unmapped the `ByteBuffer` if closed. If not closed, the `ByteBuffer` would be GC:ed.
> 
> This proposal mimics the old behavior but may hold the mapped memory region slightly longer.
> 
> Tested and passed tier1, tier2, tier3

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Move test

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15814/files
  - new: https://git.openjdk.org/jdk/pull/15814/files/9b23ecbe..e8ce5910

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15814&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15814&range=00-01

  Stats: 294 lines in 2 files changed: 147 ins; 147 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15814.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15814/head:pull/15814

PR: https://git.openjdk.org/jdk/pull/15814