Move minima/thunks to own file in gen.

This was triggered by an experiment about generating even more 'minima'
like this, such as a batch of helper structs which would hold all the
common "no, you can't do operation X on something of kind Y" methods.
However, that diff isn't here; and is probably going to turn out to be
abandoned, because after trying it out, I found that it was pretty hard
to do good error messages (which... was pretty central to the point)
with that approach.  So, more on those attempts later.
Signed-off-by: Eric Myhre <hash@exultant.us>

schema/gen/go/genMinima.go

+package gengo
+
+import (
+	"io"
+)
+
+func emitMinima(f io.Writer) {
+	emitFileHeader(f)
+
+	// Iterator rejection thunks.
+	f.Write([]byte(`
+type mapIteratorReject struct{ err error }
+type listIteratorReject struct{ err error }
+
+func (itr mapIteratorReject) Next() (ipld.Node, ipld.Node, error) { return nil, nil, itr.err }
+func (itr mapIteratorReject) Done() bool                          { return false }
+
+func (itr listIteratorReject) Next() (int, ipld.Node, error) { return -1, nil, itr.err }
+func (itr listIteratorReject) Done() bool                    { return false }
+`))
+}

schema/gen/go/gen_test.go

@@ -36,18 +36,8 @@ func TestNuevo(t *testing.T) {
 		tg.EmitNodeMethodNodeBuilder(w)
 	}
 
-	f := openOrPanic("_test/thunks.go")
-	emitFileHeader(f)
-	doTemplate(`
-		type mapIteratorReject struct{ err error }
-		type listIteratorReject struct{ err error }
-
-		func (itr mapIteratorReject) Next() (ipld.Node, ipld.Node, error) { return nil, nil, itr.err }
-		func (itr mapIteratorReject) Done() bool                          { return false }
-
-		func (itr listIteratorReject) Next() (int, ipld.Node, error) { return -1, nil, itr.err }
-		func (itr listIteratorReject) Done() bool                    { return false }
-	`, f, nil)
+	f := openOrPanic("_test/minima.go")
+	emitMinima(f)
 
 	f = openOrPanic("_test/tStrang.go")
 	emitFileHeader(f)