Changes

Summary

1. include arm64-linux;
2. prefer existing OCaml installation;
3. include arm64-linux;
4. no patchelf on macOS (undetected due to cached executables?);
5. provide opam-2.1.0 for experimentation;
6. rebuild cygwin-20211004.tar.gz;
7. include arm64-linux;
8. updated to cygwin-20211004: build again;
9. merged
10. removed pointless NEWS: both Docker/ubuntu and Cygwin provide perl by default;
11. actually use cygwin-20211002 (amending ff0ca375457c);
12. discontinued perl;
13. clarified signature;
14. proper term operation Term.dest_abs;
15. tuned;
16. tuned proofs; fewer warnings;
17. more standard binder syntax;
18. clarified 'let' syntax: avoid conflict with existing 'let' in FOL;
19. tuned;
20. tuned;
21. merged
22. removal of a redundant theorem (and white space)
23. new material from the Roth development, mostly about finite sets, disjoint famillies and partitions
24. clarified and updated for release;
25. formal comment concerning 83d2208252d1 vs. d8dc8fdc46fc;
26. more NEWS and CONTRIBUTORS;
27. clarified comments;
28. support symbol \<Parallel>, based on \bigparallel from stdmaryd (stmary10.pfb: move y=1100, scale 222%, embolden by 40 units, adjust spacing);
29. clarified dependencies;