Add a "whereis" subcommand.

"gem whereis GEMNAME [REQUIREMENT ...]" shows the base directory where a
specified gem is installed. This is considered useful for use in scripts
and editor plugins, or when one simply wants to browse a gem's installed
directory.

"gem which" does a similar job, but it is focused on locating a library
file and does not work for gems that do not provide a user loadable
library.