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

• Compiling Haskell expressions typed at the GHCi prompt
• Compiling entire modules, eg via :load

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.

Trac metadata

Trac field	Value
Version	6.10.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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