Currently users who want to use the readconcern, readpref, or writeconcern packages must currently import the core library since this is where they live. To enable users to use these packages, and enable further options work, move these packages under the mongo package.

Additionally, to ensure that we don't have multiple packages with the same name, copy the functionality of these packages into the core package. Rename as is necessary, since the core library is meant to be verbose, it's okay to have more verbose names for the types and functions in this package.