cgen: auto initialize `chan` that are struct elements (#8541)

vlib/sync/struct_chan_init_test.v

@@ -0,0 +1,14 @@
+struct Abc {
+	ch chan int
+}
+
+fn f(st Abc) {
+	st.ch <- 47
+}
+
+fn test_chan_init() {
+	st := Abc{}
+	go f(st)
+	i := <-st.ch
+	assert i == 47
+}

vlib/v/gen/c/cgen.v

@@ -5732,6 +5732,10 @@ fn (mut g Gen) type_default(typ_ table.Type) string {
 		'rune' { return '0' }
 		else {}
 	}
+	if sym.kind == .chan {
+		elemtypstr := g.typ(sym.chan_info().elem_type)
+		return 'sync__new_channel_st(0, sizeof($elemtypstr))'
+	}
 	return match sym.kind {
 		.interface_, .sum_type, .array_fixed, .multi_return { '{0}' }
 		.alias { g.type_default((sym.info as table.Alias).parent_type) }