strconv: make string.int() stop parsing on invalid digit

vlib/builtin/string_int_test.v

@@ -0,0 +1,11 @@
+fn test_common_atoi() {
+	assert "70zzz".int() == 70
+	assert "2901issue".int() == 2901
+	assert '234232w'.int() == 234232
+	assert '-9009x'.int() == -9009
+	assert '0y'.int() == 0
+	for n in -10000 .. 100000 {
+		s := n.str()+"z"
+		assert s.int() == n
+	}
+}