RFR: Make attributes mutable

[Maurizio Cimadamore]

This PR makes the attribute list associated with a declaration (and a type) mutable. This means that we can now tweak a declaration/type after the fact. While the declaration/type is still immutable, the information attached to it can grow over time. This should be useful to reduce coupling between different steps in the extraction process. I plan to use this to simplify the way we deal with name mangling.

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Commit messages:
 - Cleanup AttributedImpl
 - Add copyright
 - Make attributes mutable

Changes: https://git.openjdk.org/jextract/pull/146/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=146&range=00
  Stats: 371 lines in 12 files changed: 159 ins; 189 del; 23 mod
  Patch: https://git.openjdk.org/jextract/pull/146.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/146/head:pull/146

PR: https://git.openjdk.org/jextract/pull/146