licenses: convert license tags to SPDX

asmrefine: Add support for 64-bit architectures.

asmrefine: add field offset rewrite rules.

Currently unused, but these are useful for debugging proofs in SEAR
about struct access updates.

asmrefine: update to Isabelle2019; reduce warnings

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.

Isabelle2016-1: update references to renamed constants and facts

Try to avoid emitting const-globals via memory.

Sometimes it's simpler to access an unknown field of a const
global by just computing the offset from its symbol in memory
and assuming the relevant words are in the .rodata section. But
for known fields, it's easier to just figure out what the
constant value is. This complicates the proof slightly, since
it has to guess which case it is in.

asmrefine: 2015 udpate

Fix asmrefine stuff for 2014 branch.

SIMPL->Graph proofs largely working.

Import release snapshot.