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.

x64 abstract/machine: introduce and use FPUNullState

The FPU state is opaque, and its null state is not necessary an array of zeroes.
Instead, the null state is a snapshot taken after initialisation.

aspec/haskell/machine: refactor user_context interface

- remove separate abstract set_/get_register implementation, directly use machine op
- make interface aware that user_context does not always need to equal
(register => machine_word)
- introduce FPU state on x64

x64: got haskell translator running on existing haskell kernel

x64: add arch_split'd x64 spec with IOMMU stuff