compiler, runtime: implement delete builtin

testdata/map.go

@@ -22,6 +22,8 @@ func main() {
 	readMap(testmap1, "data")
 	readMap(testmap2, "three")
 	readMap(testmap2, "ten")
+	delete(testmap2, "six")
+	readMap(testmap2, "seven")
 }
 
 func readMap(m map[string]int, key string) {

testdata/map.txt

@@ -33,3 +33,16 @@ map read: ten = 10
   ten = 10
   eleven = 11
   twelve = 12
+map length: 11
+map read: seven = 7
+  one = 1
+  two = 2
+  three = 3
+  four = 4
+  five = 5
+  seven = 7
+  eight = 8
+  nine = 9
+  ten = 10
+  eleven = 11
+  twelve = 12