Initialise a GHC session.
If you implement a custom
GhcMonad you must call this function
in the monad run function. It will initialise the session variable and
clear all warnings.
The first argument should point to the directory where GHC's library
files reside. More precisely, this should be the output of
ghc
--print-libdir of the version of GHC the module using this API is
compiled with. For portability, you should use the
ghc-paths
package, available at
http://hackage.haskell.org/package/ghc-paths.