licenses: convert license tags to SPDX

global: isabelle update_cartouches

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.

Removes all trailing whitespaces

Isabelle2016-1: update to new ML API

Update references to renamed ML constants; supply default arguments to
functions with additional parameters; etc.

Isabelle2016: fix SimpStrategy for changes in simproc setup

lib: more 2015 update

Import release snapshot.