[ESS] cleanup ess namespace (prefix)

[Leo Liu]

Hello ESS team,

Since emacs only have one single global namespace and relying on prefix
as a means to resolve conflicts between 3rd-party libraries, I wonder if
you can take some effort to migrate the ESS packages to be properly
prefixed. Specially do not follow the practice of naming upstream. For
example, I see, get-ess-process and get-ess-buffer and tons of others in
ESS. Those are just names waiting to cause trouble.

HTH,
Leo