RFR: 8335939: Hide element writing across the ClassFile API [v5]

[Chen Liang]

> `WritableElement` has always been one of the biggest peculiarities of ClassFile API: it exposes element writing yet has no corresponding reading ability exposed. Its existence creates a lot of API noise, increasing maintenance cost in the long run. (This is an iceberg whose tip was exposed in #14705)
> 
> Removal details:
> - `LocalVariable/LocalVariableType.writeTo`
> - `WritableElement`
> - In `BufWriter`:
>   - `writeList(List<WritableElement>)`
>   - `writeListIndices`: Hidden as we are not exposing `BoundAttribute.readEntryList`
>   - `writeBytes(BufWriter other)`
> - `ClassReader.compare`: Avoid reading from `BufWriter`
> 
> Future implementation cleanup out of scope of this patch:
> - Annotation writing can be upgraded and move away from `Util.Writable`
> - The writing of CP indices and attributes can move to their dedicated methods
> - reading of entry list can pair up with writing of entry list in ClassReader/BufWriter in future addition

Chen Liang has updated the pull request incrementally with one additional commit since the last revision:

  Fix test compile errors

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20205/files
  - new: https://git.openjdk.org/jdk/pull/20205/files/5ce1943a..09a6f60a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20205&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20205&range=03-04

  Stats: 4 lines in 2 files changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/20205.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20205/head:pull/20205

PR: https://git.openjdk.org/jdk/pull/20205