<i18n dev> RFR: 8276220: Reduce excessive allocations in DateTimeFormatter [v10]

[Claes Redestad]

> Prompted by a request from Volkan Yazıcı I took a look at why the java.time formatters are less efficient for some common patterns than custom formatters in apache-commons and log4j. This patch reduces the gap, without having looked at the third party implementations. 
> 
> When printing times:
> - Avoid turning integral values into `String`s before appending them to the buffer 
> - Specialize `appendFraction` for `NANO_OF_SECOND` to avoid use of `BigDecimal`
> 
> This means a speed-up and reduction in allocations when formatting almost any date or time pattern, and especially so when including sub-second parts (`S-SSSSSSSSS`).
> 
> Much of the remaining overhead can be traced to the need to create a `DateTimePrintContext` and adjusting `Instant`s into a `ZonedDateTime` internally. We could likely also win performance by specializing some common patterns.
> 
> Testing: tier1-3

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

  Remove accidentally committed experimental @Stable (no effect on micros)

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6188/files
  - new: https://git.openjdk.java.net/jdk/pull/6188/files/f6adb5b5..b663fe63

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6188&range=09
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6188&range=08-09

  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6188.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6188/head:pull/6188

PR: https://git.openjdk.java.net/jdk/pull/6188