Corrected Kernel#Complex's signature

[sampersand]

This updates Kernel#Complex's signature.

In particular, if the first argument defines .to_c, and no second argument is supplied, it calls and returns that. (In the case where exception: false is given, all exceptions (even Exception itself) that would be thrown by .to_c are swallowed, and nil is returned.)

Two additional variants, for when exception: false is explicitly given, were added. In these cases, the return value is Complex?.

[sampersand]

Technically, there could also be an | (untyped, ?untyped, exception: false) -> nil variant, but that seems a bit awkward.

[sampersand]

I've added in the | (untyped, ?untyped, exception: false) -> nil variant for completeness

[sampersand]

Consolidated in #1445