export public-facing functions

`blabr` used to export every variable defined in its code. For various reasons, it is not a good idea so I switched to explicitly exporting functions/variables that users might use. If you encounter a function that is not exported, please add it in a comment below.

In the meantime, use `blabr:::` to access a that used to be available after `library(blabr)` and now isn't.