Allow asserting a predicate

From todo.txt:

This would be useful in a manual: ensure that the results are as expected and print a big red warning if not. For example:

:assert DEC_STRING_TO_INT("10000000000000000000000000000000000",-32)=100

This could also be used as a kind of test or for notebooks to be filled in by students, where the teacher has added the asserts at the bottom.