Idris HEAD seems to require some hints to make toState' and fromState' type-check

This patch fixes this error when using Idris from git HEAD

```
$ make
idris --build iquery.ipkg
Type checking ./IQuery/State.idr
./IQuery/State.idr:94:12:Can't convert
        State STInt
with
        State t
./IQuery/State.idr:116:9:Can't convert
        State STInt
with
        State t
make: *** [build] Error 1
```