Compile the "fold" and "simplify" passes into a signal pass

Compile the "fold" and "simplify" passes into a signal pass

Squashed commit of the following:

commit 95a1bfaa420f4273891af57e286641a287f9261c
Author: Nick Gasson <nick@nickg.me.uk>
Date:   Mon Jan 2 21:11:32 2017 +0000

    Fix regression of issue 183

commit 15229bcecf95234d8d5a5ac305cee26a48ec93ea
Author: Nick Gasson <nick@nickg.me.uk>
Date:   Mon Jan 2 19:28:31 2017 +0000

    Cleanup

commit 014d26fee68c40c9309377ad3d235c08d42acccd
Author: Nick Gasson <nick@nickg.me.uk>
Date:   Thu Dec 29 18:13:42 2016 +0000

    Remove unnecessary temporary variable

commit 60246a224c8b859d03d792142aa66f62f1402767
Author: Nick Gasson <nick@nickg.me.uk>
Date:   Thu Dec 29 18:08:44 2016 +0000

    Remove more debug prints

commit 624b870f41208c8d71d25ec9174a93b6d099fb22
Author: Nick Gasson <nick@nickg.me.uk>
Date:   Thu Dec 29 18:07:34 2016 +0000

    Remove debug prints

commit 1aa54d97359a86a617ccaf5d576f334e4df86fa2
Author: Nick Gasson <nick@nickg.me.uk>
Date:   Sun Nov 27 14:42:46 2016 +0000

    WIP merging fold and simplify phases