GHC API: add checkAndLoadModule
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.