A test runner that writes s-expressions to the default output port.

Since some expressions are not printable by default (like records),
this library offers `(add-displayable! predicate transformer)`,
which will apply `transformer` to any value that satisfies `predicate`
in that test runner. This will override any previous operation.

To use, just import `(mcgoron srfi 64)` and use `(factory)` to make a new
test runner.

There is currently no test suite for this because it would be tedious to
write.