Unify usage of "host:" prefix for host modules between Rust and NodeJS

[jprendes]

More bikeshedding: Do we want to add the "host:" prefix? If we do, should we also add it on the rust impl?

Originally posted by @jprendes in #31 (comment)

Currently host modules registered from NodeJS get an automatic "host:" prefix, while modules registered from Rust do not. We should unify the behaviour in one way or the other.