Overhaul. Most importantly, allow transparency declarations.
* Enable the RHSs of macros to explicity mark subexpressions as
transparent, thus removing (o-branch) tags from them and allowing them
to be shown on their own.
* Changed to a more explicit syntax, marking macros with uppercase
first letter and literals with a $, thereby removing the need for
macro definitions to declare the literals & macros they used.
* Simplified instantiation and matching by changing "o-branch" origins
to simple taint markers.