Rollup merge of #82130 - jhpratt:const-option-result, r=RalfJung

Rollup merge of #82130 - jhpratt:const-option-result, r=RalfJung

Make some Option, Result methods unstably const

The following methods are now unstably const:

- Option::transpose
- Option::flatten
- Result::flatten

While some methods for could likely be made `const` in the future, nearly all of them require something to be dropped at compile-time, which isn't currently supported. The functions listed above should have a trivial path to stabilization.