For uniformity of Enter.
Previously, `ServerT Raw m ~ Application`. Seems reasonable, but has the
unfortunate consequence of making `Enter` useless for `Raw` routes.
With this change `Tagged m Application` is retagged by `Enter`.
Instead of only exposing 'serveDirectory', which picks a specific static file
serving strategy (file server settings), we now expose 4 different variants each
corresponding to an variant of StaticSettings in wai-app-static.
In addition to these, we expose a more flexible 'serveDirectoryWith' function
which allows the user to specify some arbitrary StaticSettings, if the 4
existing variants do not cover a user's needs.
