Tame a few tests that relied on pairs being somewhat deterministic (#6371)

* Mangle stupid defaults test so that it compares tables, and not a non-deterministic string representation of one.

It's still extremely dumb and annoying to update. (i.e., feel free to kill it with fire in a subsequent PR, I think everybody would cheer).

* Rewrite DepGraph to be deterministic

i.e., fully array based, no more hashes, which means no more pairs randomly re-ordering stuff.

Insertion order is now preserved.

Pretty sure a couple of bugs have been fixed and/or added along the way
;p.

* Resync frontend/apps/filemanager/lib/md.lua w/ upstream

And use orderedPairs in the attribute parsing code, just to make that stupid test happy.

spec/unit/widget_menu_spec.lua

@@ -19,6 +19,7 @@ describe("Menu widget", function()
                 callback = cb2
             },
         })
+        --- @fixme: Currently broken because pairs (c.f., https://github.com/koreader/koreader/pull/6371#issuecomment-657251302)
         assert.are.same(re, {
             {
                 text = 'navi',