text/template: new, simpler API

The Set type is gone. Instead, templates are automatically associated by
being parsed together; nested definitions implicitly create associations.
Only associated templates can invoke one another.

This approach dramatically reduces the breadth of the construction API.

For now, html/template is deleted from src/pkg/Makefile, so this can
be checked in. Nothing in the tree depends on it. It will be updated next.

R=dsymonds, adg, rsc, r, gri, mikesamuel, nigeltao
CC=golang-dev
https://golang.org/cl/5415060
diff --git a/src/pkg/text/template/parse/parse_test.go b/src/pkg/text/template/parse/parse_test.go
index 5c10086..fc93455 100644
--- a/src/pkg/text/template/parse/parse_test.go
+++ b/src/pkg/text/template/parse/parse_test.go
@@ -236,7 +236,7 @@
 
 func TestParse(t *testing.T) {
 	for _, test := range parseTests {
-		tmpl, err := New(test.name).Parse(test.input, "", "", nil, builtins)
+		tmpl, err := New(test.name).Parse(test.input, "", "", make(map[string]*Tree), builtins)
 		switch {
 		case err == nil && !test.ok:
 			t.Errorf("%q: expected error; got none", test.name)