RFR: 8236142: DocTrees should provide getCharacters(EntityTree)

[Jonathan Gibbons]

A new method is added to the `DocTrees` utility class to return the characters represented by a named or numeric
entity, as represented in an `EntityTree`.

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Commit messages:
 - add DocTrees.getCharacters(EntityTree)

Changes: https://git.openjdk.java.net/jdk/pull/122/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=122&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8236142
  Stats: 4460 lines in 7 files changed: 2230 ins; 2222 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/122.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/122/head:pull/122

PR: https://git.openjdk.java.net/jdk/pull/122