RFR: 4926314: Optimize Reader.read(CharBuffer) [v7]

[Philippe Marschall]

> Implement three optimiztations for Reader.read(CharBuffer)
> 
> * Add a code path for heap buffers in Reader#read to use the backing array instead of allocating a new one.
> * Change the code path for direct buffers in Reader#read to limit the intermediate allocation to `TRANSFER_BUFFER_SIZE`.
> * Implement `InputStreamReader#read(CharBuffer)` and delegate to `StreamDecoder`.
> * Implement `StreamDecoder#read(CharBuffer)` and avoid buffer allocation.

Philippe Marschall has updated the pull request incrementally with two additional commits since the last revision:

 - Replace c-style array declarations
 - Share work buffer between #skip and #read

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1915/files
  - new: https://git.openjdk.java.net/jdk/pull/1915/files/08948f93..fc29f3e6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1915&range=06
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1915&range=05-06

  Stats: 28 lines in 1 file changed: 8 ins; 6 del; 14 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1915.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1915/head:pull/1915

PR: https://git.openjdk.java.net/jdk/pull/1915