RFR: 8305774: String.join(CharSequence, Iterable) can be optimized if Iterable is a Collection [v3]

[Tingjun Yuan]

> In the current implementation of `String.join(CharSequence, Iterable)`, the temp array `elems` is always initialized with a length of 8. It will cause many array recreations when the `Iterable` contains more than 8 elements. Furthermore, it's very common that an `Iterable` is also a `Collection`. So if the `Iterable` is an instance of `Collection`, the initial length of the array can be `((Collection<?>)elements).size()`. It will not change the current behavior even if the `Collection` is modified asynchronously.
> 
> I don't know whether this change requires a CSR request.

Tingjun Yuan has updated the pull request incrementally with one additional commit since the last revision:

  use spliterator().estimateSize()

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13383/files
  - new: https://git.openjdk.org/jdk/pull/13383/files/50e670c7..4fe345ef

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13383&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13383&range=01-02

  Stats: 9 lines in 1 file changed: 7 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13383.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13383/head:pull/13383

PR: https://git.openjdk.org/jdk/pull/13383