We can now write
```
@[init] def initFn : IO Unit := ...
```
instead of
```
def initFn : IO Unit := ...
@[init initFn] constant execInitFn : Unit := ()
```
We can use this primitive to process command line arguments of the form
`-D <key> = <value>`
TODO: allow users to attach `[init]` to definitions of the form
```
@[init] def foo : IO Unit := ...
```
and avoid the awkward auxiliary constant.