RFR: 8236142: DocTrees should provide getCharacters(EntityTree) [v2]

[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`.

Jonathan Gibbons has updated the pull request incrementally with one additional commit since the last revision:

  Update Copyright years

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/122/files
  - new: https://git.openjdk.java.net/jdk/pull/122/files/33f2e636..1d3489c8

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=122&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=122&range=00-01

  Stats: 5 lines in 4 files changed: 0 ins; 0 del; 5 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