Most unit forms are supported, including most of the "infer" forms that
infer imports/exports/linkages from the current context.
Notably, none of the structural linking forms for units are supported, and
`define-unit-binding` is also currently unsupported.
Allow more cases that are allowed for ordinary function
contracts and explicitly error instead of internal errors
for other cases.
Closes Github Issue #50
When exporting row polymorphic functions from TR, just
use absent clauses to ensure that TR won't accidentally
try to add pre-existing fields/methods. No sealing is
needed because the typechecker enforces parameteric use
of the class.
