a special `last` variable in for loops (experimental)

vlib/builtin/array_test.v

@@ -479,3 +479,17 @@ fn test_eq() {
 	assert [5,6,7].eq([6,7]) == false
 	assert [`a`,`b`].eq([`a`,`b`]) == true
 }
+
+fn test_for_last() {
+	numbers := [1, 2, 3, 4]
+	mut s := '['
+	for num in numbers {
+		s += '$num'
+		if !last {
+			s += ', '
+			
+		}	
+	}	
+	s += ']'
+	assert s == '[1, 2, 3, 4]'
+}	

vlib/compiler/parser.v

@@ -3377,6 +3377,15 @@ fn (p mut Parser) for_st() {
 		else if is_arr {
 			typ = typ[6..]// all after `array_`
 			p.gen_for_header(i, tmp, typ, val)
+			p.register_var(Var {
+				name: 'last'
+				typ: 'bool'
+				is_mut: false
+				is_used: true
+			})
+			// TODO don't generate if it's not used.
+			// Otherwise it's a C warning + perf.
+			p.genln('bool last = $i == $tmp . len - 1;')
 		}
 		else if is_str {
 			typ = 'byte'