RFR: 8317993: Add capturing factories to classes in java.util.function package [v2]

[Per Minborg]

> This PR proposes to add a number of "capturing factories" in classes in the `java.util.function` package.
> 
> The PR additionally (an optionally) proposes to add a new function `UnaryOperator::andThenUnary` to allow composition while retaining the `UnaryOperator` type.
> 
> With the new changes, it is possible to write code like this (example from `java.util.function.Function`):
> 
> 
> // Resolve ambiguity
> var function = Function.of(String::isEmpty); // Function<String, Boolean>
> var predicate = Predicate.of(String::isEmpty); // Predicate<String>
> 
> // Fluent composition
> var chained = Function.of(String::length)       // Function<String, Integer>
>                    .andThen(Integer::byteValue);      // Function<String, Byte>
> 
> 
> Please see the original bug report for a comprehensive description of these proposed changes.
> 
> Note: It is not the objective to promote `var` declaration or to prevent previous ways of capturing lambdas and method references. The comments in the code above is for explaining the binding and once that becomes obvious, such comments are likely to not appear in real code. Users that prefers having a normal type declaration can still do that.
> 
> Note: Functional interfaces of primitives have not been considered (in this round). Otherwise, functional interfaces that might be ambiguous or that supports composition have been included. Hence, `Supplier` did not get a factory method.

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

  Add co/contra variance

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16213/files
  - new: https://git.openjdk.org/jdk/pull/16213/files/dbfb4008..96586f7e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16213&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16213&range=00-01

  Stats: 34 lines in 7 files changed: 21 ins; 1 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/16213.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16213/head:pull/16213

PR: https://git.openjdk.org/jdk/pull/16213