RFR: JDK-8321545: Override toString() for Format subclasses [v3]

[Justin Lu]

> Please review this PR which implements toString() for the `Format` subclasses. Corresponding CSR: [JDK-8323088](https://bugs.openjdk.org/browse/JDK-8323088)
> 
> The general specification follows a template that provides the locale (if the class is localized) and any relevant patterns. The specification was intentionally kept minimal and deliberately worded as "for debugging".
> 
> An example of all the classes has output such as 
> 
> 
> CompactNumberFormat [locale: "English (United States)", decimal pattern: "foo#0.00#baz", compact patterns: "[, , , {one:0K other:0K}, {one:00K other:00K}, {one:000K other:000K}, {one:0M other:0M}, {one:00M other:00M}, {one:000M other:000M}, {one:0B other:0B}, {one:00B other:00B}, {one:000B other:000B}, {one:0T other:0T}, {one:00T other:00T}, {one:000T other:000T}]"]
> 
> DecimalFormat [locale: "English (United States)", pattern: "foo#0.00#baz"]
> 
> SimpleDateFormat [locale: "Chinese (China)", pattern: "EEE, MMM d, ''yy"]
> 
> ListFormat [locale: "English (United States)", start: "{0}, {1}", middle: "{0}, {1}", end: "{0}, and {1}", two: "{0} and {1}", three: "{0}, {1}, and {2}"]
> 
> MessageFormat [locale: "Chinese (China)", pattern: "foo {0}"]
> 
> ChoiceFormat [pattern: "0#foo"]

Justin Lu has updated the pull request incrementally with two additional commits since the last revision:

 - swap placement of decimal pattern and compact patterns. Expand on tests
 - add unit tests

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/17355/files
  - new: https://git.openjdk.org/jdk/pull/17355/files/1dfc949d..3157ec8e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17355&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17355&range=01-02

  Stats: 359 lines in 7 files changed: 357 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/17355.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17355/head:pull/17355

PR: https://git.openjdk.org/jdk/pull/17355