Add pilfer to Core

[vinniefalco]

Add pilfer to Core so that it can be used by more than just Boost.JSON

[Lastique]

I don't understand what's this about. Closing as spam.

@vinniefalco Please be clear about the problem description in future issues and avoid jargonisms.

[pdimov]

It's a request to add pilfer to Core. pilfer currently exists in Boost.JSON detail, and has been proposed by me in P0308.

That'd actually be a legitimate addition to Core, for a change - the component is already present in another library and is needed as common infrastructure in others.

[vinniefalco]

See: https://github.com/boostorg/json/blob/98d37f93706b50fd057d14bd241a2bfbcfec1527/include/boost/json/pilfer.hpp#L27

[breese]

You may want to reconsider the name. This facility is a optimization on move semantics, and is thus special-purpose and less widely used. In such cases we should consider a longer, more expressive name, such as semi_destructive_move(), to make it more likely that arbitrary users understand the code without having to read up on "pilfering".

[vinniefalco]

I like pilfer