Add CopyVec and CopyString

[sffc]

Issue #230 was to make Vec and String implement Copy. However, as explained by @MarcusGrass, @delta4chat and others in #230, it is not possible to conditionally implement Copy on Vec if Vec is to allow a custom Drop impl, which the team wants.

An alternative suggested by @nickray and @japaric was to introduce a type CopyVec. This would provide an avenue to CopyString. The motivation for a Copy String is still the same as it was in #230, so I hope the team can explore other avenues.