compiler: fix initialization of struct fields, fix #1164

Make `s{}` initializers work. Deduplicate code a bit along the way. The test
added follows bf6aa02dcf test.

pkg/compiler/struct_test.go

@@ -405,6 +405,29 @@ var structTestCases = []testCase{
 		}`,
 		big.NewInt(12),
 	},
+	{
+		"uninitialized struct fields",
+		`package foo
+               type Foo struct {
+                       i int
+                       m map[string]int
+                       b []byte
+                       a []int
+                       s struct { ii int }
+               }
+               func NewFoo() Foo { return Foo{} }
+               func Main() int {
+                       foo := NewFoo()
+                       if foo.i != 0 { return 1 }
+                       if len(foo.m) != 0 { return 1 }
+                       if len(foo.b) != 0 { return 1 }
+                       if len(foo.a) != 0 { return 1 }
+                       s := foo.s
+                       if s.ii != 0 { return 1 }
+                       return 2
+               }`,
+		big.NewInt(2),
+	},
 }
 
 func TestStructs(t *testing.T) {