Retain CAFs in the interpreter

iserv-proxy-interpreter dynamically loads libraries. It must retain CAFs
to avoid issues like:

> iserv-proxy-interpreter.exe:internal error:Evaluated a CAF that was GC'd!

ghc-lib uses a hacky keepCAFsForGHCi.c file; iserv still uses a custom
hs-main C file. We use -fkeep-cafs as it has the same effect.

Author: hsyl20

iserv-proxy.cabal

@@ -99,6 +99,9 @@ Executable iserv-proxy
 Executable iserv-proxy-interpreter
    Default-Language: Haskell2010
    Main-Is: Interpreter.hs
+   -- We need to pass -fkeep-cafs to ensure that the interpreter retains CAFs
+   -- Iserv and GHC do something similar.
+   ghc-options: -fkeep-cafs
    Build-Depends: base       >= 4   && < 5,
                   iserv-proxy