Make compatible with reflection API in agda stdlib HEAD

Some fancy refactoring was made in the development version of Agda's standard library, as you can see here:

https://github.com/agda/agda-stdlib/commit/441df785b9867bfeb25fe20f71472f2a41844ea1#diff-ada01090d397038ded8e164cc554bbc3R74

I like to live on the bleeding edge, but I guess lots of other people also run some form of stdlib from the devel repo. Besides, future versions will probably be refactored like the HEAD is now, so it's good to be compatible :)

Maybe you don't want to pull this into your master, but a "stdlib-devel" branch could be nice...