Improve projection language

 * simple operators like + and * for primitives
 * string interpolation
 * a map like operator that will take only a lambda defined in place (no user defined functions), and can take an optionnaly defined item separator

```
${projections.collectToString("${it.hostname}-${it.id+10}",",")}
```

should return

```
"host1-110","host2-111","host3-112"
```