Add the IRContext (part 1)

Add the IRContext (part 1)

This is the first part of adding the IRContext.  This class is meant to
hold the extra data that is build on top of the module that it
owns.

The first part will simply create the IRContext class and get it passed
to the passes in place of the module.  For now it does not have any
functionality of its own, but it acts more as a wrapper for the module.

The functions that I added to the IRContext are those that either
traverse the headers or add to them.  I did this because we may decide
to have other ways of dealing with these sections (for example adding a
type pool, or use the decoration manager).

I also added the function that add to the header because the IRContext
needs to know when an instruction is added to update other data
structures appropriately.

Note that there is still lots of work that needs to be done.  There are
still many places that change the module, and do not inform the context.
That will be the next step.