Rest-core: remove dependency on cgi and multipart.

The cgi dependency causes problems with GHC 7.8. Since we use only a very small part of the multipart stuff, it's easier to just copy it in here.
Rest-gen: raise 'process' upper bound.
Rest-gen: generate hash with 'hashable'.

The Data.Hashtable module was removed in GHC 7.8.
Rest-gen: System.Cmd -> System.Process.

System.Cmd is deprecated.
Bump rest-core and rest-gen.
Revert "Rest-core: remove dependency on cgi and multipart."

This reverts commit 0b4f7fcf1336dc5ab9d5ea878170a6b1a65d297e.

We need the parsing functionality as well.
Remove dependency on cgi.
Bump rest-core.