Further testing

[eric-unc]

Thanks to #24, there a lot of unit tests, but there are more that could be added.

• and tests. Some errors are currently resolved in parsing rather than evaluation, and personally I think this is a mistake. Waiting on Create independent scanner/parser #2.
• or tests. ditto.
• tests for vals.rs.
• tests for the scanner/parser.
• if tests.
• cond tests.
• define tests.
• exit tests.
• tests for io.
• list-each
• More procedure tests.

Some of these are more important than others, of course.