RFR: 8335939: Hide element writing across the ClassFile API

[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

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Commit messages:
 - 2 test failures
 - Web review cleanup
 - Remove WritableElement and reduce Writable usage
 - Fix up usages of Util.write
 - Hide writeTo from all class file elements

Changes: https://git.openjdk.org/jdk/pull/20205/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20205&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335939
  Stats: 456 lines in 48 files changed: 98 ins; 197 del; 161 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