Update mappings to use the when construct for guards

[pmundkur]

TODO after the next Sail compiler release.

See rems-project/sail#1106

[arichardson]

This would make a lot of the clauses much easier to read - hopefully we can get a new release soon @Alasdair?

[Alasdair]

Yes, fixing that and the outstanding warnings about incomplete patterns were some of the last few things I wanted to get done. I think it's just improving some of the configuration system error messages and it should be good.