Add a `notmissing` function.

[pdeffebach]

With broadcasting, .!ismissing.(x) is awkward to write. It would be nice to have a function notmissing that would get around this.

[nalimilan]

It looks a bit funny, but duplicating functions doesn't sound great...

[ararslan]

Also note that there are other ways you can write it depending on the end goal. A direct replacement would be map(!ismissing, x), but to index into a structure, filtering out missing values, you could get indices with findall(!ismissing, x), or use filter if an appropriate method exists for your structure.