parser: fix mutability with chained fields

compiler/tests/repl/chained_fields.repl

@@ -0,0 +1,78 @@
+/*  1 */ struct A { mut: v int }
+/*  2 */ struct B {      a A }
+/*  3 */ struct C { mut: b B }
+/*  4 */ struct D { mut: c C }
+
+/*  5 */ struct E { mut: v []int }
+/*  6 */ struct F {      e []E }
+
+/*  7 */ mut s := 'hello world'
+/*( 8)*/ s.len = 0   // Error (field len immutable)
+
+/*  8 */ mut b := B{}
+/*( 9)*/ b.a.v = 1   // Error (field a immutable)
+/*( 9)*/ b.a = A{}   // Error (field a immutable)
+/*  9 */ b = B{A{2}} // Correct
+
+/* 10 */ mut c := C{}
+/* 11 */ c.b = B{}   // Correct
+/*(12)*/ c.b.a = A{} // Error (field a immutable)
+/*(12)*/ c.b.a.v = 1 // Error (field a immutable)
+
+/* 12 */ c2 := C{}
+/*(13)*/ c2.b = B{}  // Error (c2 immutable)
+/* 13 */ mut d := D{}
+/* 14 */ d.c.b = B{} // Correct
+
+/* 15 */ mut f := F{}
+/*(16)*/ f.e << E{}      // Error (field e immutable)
+/*(16)*/ f.e[0].v << 1   // Error (field e immutable)
+
+/* 16 */ e := E{}
+/*(17)*/ e.v << 1    // Error (e immutable)
+
+===output===
+.vrepl_temp.v:8:14: cannot modify immutable field `len` (type `string`)
+declare the field with `mut:`
+struct string {
+  mut:
+	len int
+}
+.vrepl_temp.v:9:14: cannot modify immutable field `a` (type `B`)
+declare the field with `mut:`
+struct B {
+  mut:
+	a A
+}
+.vrepl_temp.v:9:12: cannot modify immutable field `a` (type `B`)
+declare the field with `mut:`
+struct B {
+  mut:
+	a A
+}
+.vrepl_temp.v:12:14: cannot modify immutable field `a` (type `B`)
+declare the field with `mut:`
+struct B {
+  mut:
+	a A
+}
+.vrepl_temp.v:12:16: cannot modify immutable field `a` (type `B`)
+declare the field with `mut:`
+struct B {
+  mut:
+	a A
+}
+.vrepl_temp.v:13:15: `c2` is immutable.
+.vrepl_temp.v:16:12: cannot modify immutable field `e` (type `F`)
+declare the field with `mut:`
+struct F {
+  mut:
+	e []E
+}
+.vrepl_temp.v:16:17: cannot modify immutable field `e` (type `F`)
+declare the field with `mut:`
+struct F {
+  mut:
+	e []E
+}
+.vrepl_temp.v:17:17: `e` is immutable (can't <<)