add FixKw, a companion to Fix{N}

[matthias314]

Keyword arguments are mentioned several times in the discussion of the recent Fix{N} PR #54653. However, support for them was dropped in the end. I don't know if that was to keep Fix{N} simple or for more fundamental reasons. I would like to propose FixKw as a way to do for keyword arguments what Fix{N} does for positional arguments. I think it should exist for the sake of completeness. It is also helpful because map, reduce and the like don't allow to pass keyword arguments to the function one uses.

I've chosen the name FixKw to resemble Fix although the "fixed" keyword arguments can still be overridden. I'm open to other names.

Please let me know what you think. If you like it, I can add tests and update NEWS.md.

[Roger-luo]

Should this extend Fix instead of having a new struct? E.g if we consider the senario of fixing kwargs it's also possible to fix some args and some kwargs.

[matthias314]

That's of course debatable. However, if one wants to fix two positional arguments, then there is also no functionality at present to combine them. FixKw allows to combine several keyword arguments because that's not more complicated than a single one.

[matthias314]

Are you interested in moving forward with this?

I think that there should be some functionality to fix keyword arguments, be it this proposal or some other solution.

[nsajko]

Fixes #59375