Stdlib: use assertEqual in all doc strings

[cknitt]

As the examples from the doc strings are now tested during CI build (or via node scripts/test.js -docstrings), we are using assertEqual in the examples to verify the results of the various stdlib function calls, e.g.,

Array.make(~length=3, #apple)->assertEqual([#apple, #apple, #apple])

However, we are not doing this consistently in all modules. E.g., it is missing in the List and Math modules.

Remark: IMHO, for documentation purposes, the assertEqual in the examples doesn't actually look that nice. But we require it for testing. Any ideas how to improve this are welcome.