compiler: implement operations on nil hashmaps

* comparing a map against nil
  * getting the length of a nil map

testdata/map.go

@@ -26,6 +26,11 @@ func main() {
 	readMap(testmap2, "seven")
 	lookup(testmap2, "eight")
 	lookup(testmap2, "nokey")
+
+	// operations on nil maps
+	var nilmap map[string]int
+	println(m == nil, m != nil, len(m))
+	println(nilmap == nil, nilmap != nil, len(nilmap))
 }
 
 func readMap(m map[string]int, key string) {

testdata/map.txt

@@ -48,3 +48,5 @@ map read: seven = 7
   twelve = 12
 lookup with comma-ok: eight 8 true
 lookup with comma-ok: nokey 0 false
+false true 2
+true false 0