Stabilisation of `result_expect`

[thepowersgang]

Introduced in RFC1119, covers the Result::expect method.

An easy candidate for stabilisation as far as I can see, as it's just a single method that brings Result's and Option's APIs closer.

(#26868)

[johalun]

When can we expect this to be usable from stable 1.2?

[alexcrichton]

This'll serve as the tracking issue for the result_expect feature

[thepowersgang]

@yohanesu Hopefully it will be stabilised for 1.4 (i.e. when current nightly is made stable)

[alexcrichton]

This feature is now entering its final comment period for stabilization.

[retep998]

Super useful function, I'm okay with stabilizing this.