bug#25929: 25.2; map-delete doesn't delete permanently 1st alist elt

[Stefan Monnier]

>> A better option might be to provide a map-remove which works
>> functionally (i.e. doesn't modify its argument by side-effects)
> I'd love to have that, but what about performance issues with
> hash-tables?

I was thinking of providing map-remove additionally to map-delete (and
make map-delete signal an error when it can't do its job).  IOW declare
that some parts of the API can't be used with all types.


        Stefan