Add support for partial reloads in the GHC API.

This is mainly for VS: when editing a file you don't want to
continually reload the entire project whenever the current file
changes, you want to reload up to and including the current file only.
However, you also want to retain any other modules in the session that
are still stable.

I added a variant of :reload in GHCi to test this.  You can say
':reload M' to reload up to module M only.  This will bring M up to
date, and throw away any invalidated modules from the session.