lib/sep_algebra: Isabelle2020 ROOT update

Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>

licenses: convert license tags to SPDX

lib: sep_tactics cleanup; session cleanup + organisation

lib: automation for separation logic & folds

lib+sysinit: add extended separation algebra and forward reasoning tactics

Added Extended Separation Algebra, which contains septraction and
separating coimplication.
Added Sep_Forward which contains tactics for reasoning forwards in
separation logic, updated sep_cancel.
Updated SysInit proofs.

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.

sep_algebra: add ROOT + regression test

Give sep_algebra its own regression test session so the examples
don't bitrot. Also fixed sep_subst example which had bitrotted.

Closes VER-515