add approximation factors in build schedule to estimate build times more conservatively;

merged

Type class patch suggested by Achim Brucker, plus tidied lemma

rearranged and reformulated abstract classes for bit structures and operations

Three new lemmas

tuned proof: avoid z3 to make it work on arm64-linux;

update to jdk-21.0.2; enforce rebuild of Isabelle/Scala + Isabelle/ML;

make build process state protected to avoid copying in subclasses (e.g. for database connections);

add build_sync tag to sync certain options (e.g., build_engine) across build processes;

clarified Mercurial version: presumably the last version that supports both python2 and python3;

more robust: avoid crash on non-Linux systems;

clarified webserver names;

proper Apache.php_name;

proper packages for mercurial_setup on Ubuntu 22.04: building from source provides hgweb modules, and also provides a defined version (6.1.1 is also provided by Ubuntu 22.04);