define-unit with the application of a unit contract. So you can think
of it as a define/contract for units that keeps the static info needed
for link inference.
svn: r13584
makes sense or not, we'll see. Easy enough to set up, and it also sets
up the plan for a new form I'd like to add, which is why I've factored out
the syntax classes into their own file.
svn: r13570