Fix flaggery for RULES (cf Trac #2497)

Fix flaggery for RULES (cf Trac #2497)

This patch executes the plan described in the discussion in Trac #2497.
Specficially:

    * Inside a RULE, switch on the forall-as-keyword in the lexer,
      unconditionally. (Actually this is done by an earlier patch.)

    * Merge the -XScopedTypeVariables and -XPatternSignatures flags,
      and deprecate the latter. Distinguishing them isn't senseless,
      but it's jolly confusing.

    * Inside a RULE, switch on -XScopedTypeVariables unconditionally.

    * Change -frewrite-rules to -fenable-rewrite-rules; deprecate the former.
      Internally the DynFlag is now Opt_EnableRewriteRules.

There's a test in typecheck/should_compile/T2497.hs