If and While should support inserts in their conditions

If and While should support inserts in their conditions. For instance this program:
```groovy
install {
  start main
}

String MY_CONSTANT = "this is a constant"

impulse process main {
  if: /say ${MY_CONSTANT}
  then {
    /say then
  }
}
```
yields the following output:
```
[@] ${MY_CONSTANT}
[@] then
```