Address #47: More xrefs for with-compilation-unit

The macro @macref previously just used @code. Change it to be @ref to generate a cross-reference instead. Replace many uses of @code{with-compilation-unit} with @macref. To make this all work, we need to add an anchor for with-compilation-unit.

Also add a new macro @keyref to create a cross reference to a keyword that prints the text as a keyword. Use this new macro to create references for :context-declarations.

Finally, the cross-references for context-declaration would take you to the line following the section. Move the anchor above the section so that you can see the section heading when navigating to the link.