Add Result.orElseWith to FSharp.Core

[T-Gro]

Option and voption modules have orElseWith function.
Result module does not.

Add it to FSharp.Core project as a function which is inlined and also uses inlineiflambda attribute for the lambda parameter passed.
Add it to implementation .fs file as well as to the .fsi signature file.
Create a rich XML documentation in the .fsi signature file.

Make sure to add rich suite of tests into FSharp.Core.UnitTests projects, following tests which exist for orElseWith already.

Add the tests in the location which corresponds to tests added to the Result module.