Check for sideffects from invalid manipulations.

Interestingly, the codegen'd map ends up doing this in a different way
than the generic map: since it *is* actually wrapping and delegating to
another assembler to handle the child value, it has to make sure *that*
is invalidated; otherwise the fact we hand calls to the child assembler
right away before doing our 'flush' would be problematic.

Splitting the 'flush' in half would also work, of course.  At present
it seems like six of one and half a dozen of the other.  We can subject
this to microbenchmarking later if it seems relevant.

_rsrch/nodesolution/impls/map_K_T.go

@@ -386,8 +386,7 @@ func (ma *_Map_K_T__Assembler) AssembleDirectly(k string) (ipld.NodeAssembler, e
 	l := len(ma.w.t)
 	ma.w.t = append(ma.w.t, _Map_K_T__entry{k: K{k}})
 	ma.w.m[K{k}] = &ma.w.t[l].v
-	// Init the value assembler with a pointer to its target and to whole 'ma' and yield it.
-	ma.va.ma = ma
+	// Init the value assembler with a pointer to its target and yield it.
 	ma.va.ca.w = &ma.w.t[l].v
 	return &ma.va, nil
 }
@@ -412,8 +411,7 @@ func (ma *_Map_K_T__Assembler) AssembleValue() ipld.NodeAssembler {
 		panic("misuse")
 	}
 	ma.state = maState_midValue
-	// Init the value assembler with a pointer to its targetand to whole 'ma' and yield it.
-	ma.va.ma = ma
+	// Init the value assembler with a pointer to its target and yield it.
 	ma.va.ca.w = &ma.w.t[len(ma.w.t)-1].v
 	return &ma.va
 }
@@ -500,9 +498,10 @@ func (mva *_Map_K_T__ValueAssembler) flush() {
 	//  A) the appropriate time to do that would've been *before* assignments;
 	//  A.2) accordingly, we did so before exposing this value assembler at all; and
 	//  B) if we were in a wrong state because someone holds onto this too long,
-	//   the invalidation we're about to do on `mva.ma` will make .
+	//   the invalidation we're about to do on `mva.ca.w` will make it impossible
+	//    for them to make changes in appropriately.
 	mva.ma.state = maState_initial
-	mva.ma = nil // invalidate self to prevent further incorrect use.
+	mva.ca.w = nil
 }
 func (_Map_K_T__ValueAssembler) Style() ipld.NodeStyle { panic("later") }
 

_rsrch/nodesolution/impls/map_test.go

@@ -174,6 +174,16 @@ func CheckMapStrInt(t *testing.T, ns ipld.NodeStyle) {
 			va.AssignInt(2)
 			t.Fatal("must not be reached")
 		}()
+
+		// ... and neither of these should've had visible effects!
+		wish.Wish(t, ma.Finish(), wish.ShouldEqual, nil)
+		n := nb.Build()
+		wish.Wish(t, n.Length(), wish.ShouldEqual, 1)
+		v, err := n.LookupString("whee")
+		wish.Wish(t, err, wish.ShouldEqual, nil)
+		v2, err := v.AsInt()
+		wish.Wish(t, err, wish.ShouldEqual, nil)
+		wish.Wish(t, v2, wish.ShouldEqual, 1)
 	})
 	t.Run("builder reset works", func(t *testing.T) {
 		// TODO