licenses: convert license tags to SPDX

lib: automation for separation logic & folds

globally use session-qualified imports; add Lib session

Session-qualified imports will be required for Isabelle2018 and help clarify
the structure of sessions in the build tree.

This commit mainly adds a new set of sessions for lib/, including a Lib
session that includes most theories in lib/ and a few separate sessions for
parts that have dependencies beyond CParser or are separate AFP sessions.
The group "lib" collects all lib/ sessions.

As a consequence, other theories should use lib/ theories by session name,
not by path, which in turns means spec and proof sessions should also refer
to each other by session name, not path, to avoid duplicate theory errors in
theory merges later.

wp_cleanup: update proofs for new wp behaviour

The things that usually go wrong:
- wp fall through: add +, e.g.
apply (wp select_wp) -> apply (wp select_wp)+

- precondition: you can remove most hoare_pre, but wpc still needs it, and
sometimes the wp instance relies on being able to fit a rule to the
current non-schematic precondition. In that case, use "including no_pre"
to switch off the automatic hoare_pre application.

- very rarely there is a schematic postcondition that interferes with the
new trivial cleanup rules, because the rest of the script assumes some
specific state afterwards (shouldn't happen in a reasonable proof, but
not all proofs are reasonable..). In that case, (wp_once ...)+ should
emulate the old behaviour precisely.

Isabelle2016-1: update references to renamed constants and facts

sep_algebra: cleanup

- fixed legacy warnings
- fixed almost all other warnings as well
- clarified some monoid lemmas that had misleading mixed notation
- improved layout style compliance (seriously, people..)
- comment typo fixes etc

lib/sep_algebra: 2015 update

CapDL: Made IRQ Nodes a new object type, not a small CNode.

IRQ Nodes are now their own object type in capDL. This makes it much easier
to distinguish between "real" CNodes and IRQ Nodes.

Updated:
* the capDL refinement,
* the access proofs, and
* the system initialiser.

Import release snapshot.