Does what the name suggests: it performs the function of both
checkModule and load on that module, avoiding the need to process each
module twice when checking a batch of modules.  This will make Haddock
and ghctags much faster.

Along with this is the beginnings of a refactoring of the HscMain
interface.  HscMain now exports functions for separately running the
parser, typechecher, and generating ModIface and ModDetails.
Eventually the plan is to complete this interface and use it to
replace the existing one.