Introduce Kernel#not_nil!

[Morriar]

This PR introduces 2 methods:

• Kernel#not_nil! which raises if self is nil and returns self otherwise
• Kernel#not_nil which allows the user to specify the return value with a block in case self is nil

This feature was discussed https://bugs.ruby-lang.org/issues/17326 and the general consensus seems to be that the method is useful and should be named not_nil!.

cc. @jez, @mame, @soutaro

[halostatue]

I do not believe that there is consensus on the name or functionality except that must! is unacceptable to matz. The last communication from matz on this said:

With a different name, there may be a chance for the method.

This should probably be reopened for discussion in the original ticket and put on the dev meeting prior to adding these methods. There is substantial opposition to this from multiple observers and substantial support from multiple observers with what I consider to be tepid support from Matz.

(I am personally against this in any form, whether must! or not_nil! and will reject its use from any codebase where I have such control. This can be better provided by a gem such that it is wholly opt-in for people who want to have this functionality.)

[XrXr]

Closing since the proposal was rejected. https://bugs.ruby-lang.org/issues/17326#note-21