licenses: convert license tags to SPDX

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.

autocorres: update example theories to Isabelle2016.

autocorres: Fix "DataStrctures.thy" to avoid namespace clash in Isabelle 2014.

"curr" is now taken; replace with "current".

some of the global Isabelle2014 renames

option_case -> case_option
sum_case -> case_sum
prod_case -> case_prod
Option.set -> set_option
Option.map -> map_option
option_rel -> rel_option
list_all2_def -> list_all2_iff
map.simps -> list.map
tl.simps -> list.sel(2-3)
the.simps -> option.sel

Import release snapshot.