Avoid need to have devout loaded

[thomasp85]

Currently, for reasons I can't fully understand, devout has to be loaded and attached, meaning packages using it needs to have devout in their Depends field. This may be related to #4, but I don't fully understand the dispatch so I'm not sure

[coolbutuseless]

Explanation: Within Rcpp, I was asking for the package using the equivalent of as.environment("package:devout"). I've now changed this to the equivalent of asNamespace('devout').

This seems to fix the issue of having to have {devout} loaded and attached.

Please re-open if this is still an issue.