[Kernel] Tweak signatures for conversion methods, and add unit tests in

[sampersand]

This PR tweaks some of the Kernel conversion methods' signatures, as well as cleaning up/adding relevant tests.

Of particular note is that the numeric conversions (Integer/Float/Complex/Rational)'s final signature (the (untyped, ?exception: bool) -> type? one) now marks the exception: bool as required.

[ksss]

@sampersand Thank you for your request. However, due to the extensive number of changes, I am finding it difficult to review. Would it be possible to split the changes into several Pull Requests?