Slice tricks with experimental packages

Attention !!! This subrepository ("golang.org/x/exp/slices") holds experimental and deprecated (in the old directory) packages.
The idea for this subrepository originated as the pkg/exp directory of the main repository, but its presence there made it unavailable to users of the binary downloads of the Go installation.
The subrepository has therefore been created to make it possible to go get these packages.
Warning: Packages here are experimental and unreliable.
Some may one day be promoted to the main repository or other subrepository, or they may be modified arbitrarily or even disappear altogether.
In short, code in this subrepository is not subject to the Go 1 compatibility promise. (No subrepo is, but the promise is even more likely to be violated by go.exp than the others.)