RFR: 8236142: DocTrees should provide getCharacters(EntityTree)

[Jonathan Gibbons]

On Thu, 10 Sep 2020 22:53:42 GMT, Jonathan Gibbons <jjg at openjdk.org> wrote:

> 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`.

Note that as well as the simple move for the `Entity.java` file, the init for the class is optimized to be
significantly smaller byte codes. That particular change was automated: there should be no change to the set of strings
that are accepted by the class.

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