Add cabal bindist

Add a new command `cabal bindist` that would create a unit that can be distributed (e.g. create a directory containing stripped executables and data files) to e.g. servers and/or users. `bindist` would essentially just invoke `copy` with the correct arguments.

/cc @dcoutts

Start at the existing `cabal copy` command and its command-line entry point, since the issue says `bindist` should invoke `copy` with different arguments. Determine the correct arguments and distribution layout for stripped executables and data files, then verify that `cabal bindist` produces a deployable directory.

Written by the indexing model from the issue text.