Proposal: add 'findLess' and variants to containers

[Johan Tibell]

Hi,

First, thanks for writing such a detailed proposal.

I don't have strong feelings one way or the other. Like always I'm
worried about API growth. How about performance? Can these functions
be implemented efficiently outside the library?

Since I don't feel strongly either way I'm fine with this if Milan and
the community are.

Cheers,
Johan