Find a good design for Sample

[flyingmutant]

Sample was added in ce7c277b399941e9dc949893fe405610004dbcc2, however there are several problematic things:

• as of go1.18, compiler can't allocate the result on the stack, even when Sample was inline and the k is constant (!); see:
  • https://github.com/golang/go/issues/20533
  • https://github.com/golang/go/issues/27625
  • however, changing the signature introduces inconsistency with Perm
• the map is not reused between different calls
  • however, placing it into Rand increases its size, and introduces a pointer in a previously pointer-free struct
• Floyd's sampling algorithm does not guarantee random order, so people are required to read the docs and don't forget to Shuffle if required
• potential PartialShuffle is much faster and guarantees random order (although it requires mutable slice of size n)
• having all 4 of Perm, Sample, PartialShuffle and Shuffle is maybe too much

On the other hand, Sample runs in O(k) for both time and memory. Also, algorithm is simple but non-obvious (a bit like Shuffle), as is the linear-search-instead-of-map optimization.

Also, with #1, Sample may be not required at all.