Make GHCi have separate flags for interactive Haskell expressions

It's becoming clear that in GHCi we want to have a separate set of command-line flags for

Examples of this need are:

We already have an ad-hoc difference in the way that type-class defaults are applied: http://www.haskell.org/ghc/docs/latest/html/users_guide/interactive-evaluation.html\#extended-default-rules
In #3202 (closed) there's a well-argued request to change behaviour of the monomorphism restriction

So the new feature would consist of:

The InteractiveContext should contain a set of DynFlags used for compiling command-line Haskell expressions (the interactive DynFlags)
The :set command would change both the DynFlags maintained by GHCi for compiling entire modules (the batch DynFlags), and the interactive DynFlags.
A new :seti command to change the interactive flags
Just possibly a :setb command to set the batch flags but leave the interactive ones unchanged.

Arguably it'd be good to have a command to display the current flag settings (of either DynFlags) but that's another blob of work.

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