RFR: 7903145: Cleanup TreeMaker

[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.

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Commit messages:
 - Merge branch 'master' into cleanup_treemaker
 - Initial push

Changes: https://git.openjdk.java.net/jextract/pull/13/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jextract&pr=13&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903145
  Stats: 113 lines in 2 files changed: 47 ins; 34 del; 32 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