schema compiler: example of how migrating tests to new compiler will look.

This'll probably be a lot of grind to refactor, but otherwise isn't
a huge jump semantically.

I waffled on sprinting straight ahead to writing schema DMT as JSON,
and using those as the test fixture input.  On the one hand: it would
be nice to get that much closer to purely textual test fixtures.
On the other hand, it's really verbose, and I don't want to (long run,
we'll use schema DSL for this, and while we'll also save the DMT
fixtures, they should ideally probably mostly be generated and just
human-checked rather than human-penned);
and also, since we don't have implicits implemented correctly yet,
we'd need to update all that JSON again anyway once that feature is
complete... which pushes it overall to a net "nah".

schema/gen/go/testLists_test.go

@@ -14,16 +14,17 @@ import (
 func TestListsContainingMaybe(t *testing.T) {
 	t.Parallel()
 
-	ts := schema.TypeSystem{}
-	ts.Init()
+	tsc := schema.Compiler{}
+	tsc.Init()
 	adjCfg := &AdjunctCfg{
 		maybeUsesPtr: map[schema.TypeName]bool{},
 	}
-	ts.Accumulate(schema.SpawnString("String"))
-	ts.Accumulate(schema.SpawnList("List__String",
-		"String", false))
-	ts.Accumulate(schema.SpawnList("List__nullableString",
-		"String", true))
+	tsc.TypeString("String")
+	tsc.TypeList("List__String",
+		"String", false)
+	tsc.TypeList("List__nullableString",
+		"String", true)
+	ts := tsc.MustCompile()
 
 	test := func(t *testing.T, getPrototypeByName func(string) ipld.NodePrototype) {
 		t.Run("non-nullable", func(t *testing.T) {