Better names for convertion functions from `gset` and `coPset`.

- Rename `gmap.to_gmap` into `gset_to_gmap`.
- Rename `gmap.of_gset` into `gset_to_propset`.
- Rename `coPset.to_Pset` into `coPset_to_Pset`.
- Rename `coPset.of_Pset` into `coPset_to_gset`.
- Rename `coPset.to_gset` into `coPset_to_gset`.
- Rename `coPset.of_gset` into `gset_to_coPset`.

The following `sed` script can be used for the first rename:

```
sed -i 's/to\_gmap/gset\_to\_gmap/g' $(find ./theories -name \*.v)
```

The latter is context sensitive, so was done manually.