hash structs via the bundled and altered typeutil.hash

As spotted by the protobuf package via check-third-party.sh,
the two structs below are identical:

    type alias1 = int64
    type Struct1 struct { Alias alias1 }

    type alias2 = int64
    type Struct2 struct { Alias alias2 }

Our previous approach with stripStructTags dealt with struct tags,
but it did not deal with aliases, which are now present in go/types
thanks to the new alias tracking.

The new approach properly ignores struct tags and unaliases any
type aliases, resulting in correct hashes of any type.

testdata/script/implement.txtar

@@ -1,4 +1,5 @@
-exec garble build
+go build
+exec garble -debugdir=/tmp/test build
 exec ./main
 cmp stdout main.stdout
 
@@ -45,12 +46,15 @@ var _ privateInterface = T("")
 
 type StructUnnamed = struct {
 	Foo int
-	Bar struct {
+	Bar []*struct {
 		Nested *[]string
+		NestedTagged string
+		NestedAlias int64 // we can skip the alias too
 	}
 	Named lib3.Named
 	lib3.StructEmbed
 	Tagged string // no field tag
+	Alias int64   // we can skip the alias too
 }
 
 var _ = lib1.Struct1(lib2.Struct2{})
@@ -83,14 +87,19 @@ import "test/main/lib3"
 
 type Struct1 struct {
 	Foo int
-	Bar struct {
+	Bar []*struct {
 		Nested *[]string
+		NestedTagged string `json:"tagged1"`
+		NestedAlias alias1
 	}
 	Named lib3.Named
 	lib3.StructEmbed
 	Tagged string `json:"tagged1"`
+	Alias alias1
 }
 
+type alias1 = int64
+
 -- lib2/lib2.go --
 package lib2
 
@@ -98,13 +107,18 @@ import "test/main/lib3"
 
 type Struct2 struct {
 	Foo int
-	Bar struct {
+	Bar []*struct {
 		Nested *[]string
+		NestedTagged string `json:"tagged2"`
+		NestedAlias alias2
 	}
 	Named lib3.Named
 	lib3.StructEmbed
 	Tagged string `json:"tagged2"`
+	Alias alias2
 }
+
+type alias2 = int64
 -- lib3/lib3.go --
 package lib3