RFR: 7903145: Cleanup TreeMaker [v2]

[Maurizio Cimadamore]

> This patch flattens the code in TreeMaker. The code uses several functional interfaces to abstract over the fact that a certain "tree" has to be created with or without a layout. This patch makes the code simpler, by adding general ways to construct scoped and variable declarations (e.g. from a kind) and by dropping the functional interfaces/method reference factory indirection.

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Address review comments

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Changes:
  - all: https://git.openjdk.java.net/jextract/pull/13/files
  - new: https://git.openjdk.java.net/jextract/pull/13/files/e338c07b..84aa3d53

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jextract&pr=13&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jextract&pr=13&range=00-01

  Stats: 16 lines in 1 file changed: 0 ins; 2 del; 14 mod
  Patch: https://git.openjdk.java.net/jextract/pull/13.diff
  Fetch: git fetch https://git.openjdk.java.net/jextract pull/13/head:pull/13

PR: https://git.openjdk.java.net/jextract/pull/13