RFR: 8325340: Add ASCII fast-path to Data-/ObjectInputStream.readUTF [v2]

[Claes Redestad]

> Adding a fast-path for ASCII-only modified UTF-8 strings deserialied via Data- and ObjectInputStream
> 
> Testing: tier1-3

Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:

  Refactor to use a shared impl for slow-path mod-UTF8 decoding

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17734/files
  - new: https://git.openjdk.org/jdk/pull/17734/files/90a805bf..e6a8c312

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17734&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17734&range=00-01

  Stats: 94 lines in 2 files changed: 13 ins; 46 del; 35 mod
  Patch: https://git.openjdk.org/jdk/pull/17734.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17734/head:pull/17734

PR: https://git.openjdk.org/jdk/pull/17734