A Buffer that is stable.

[matthewhammer]

Continuing and refocusing the discussion from #296, where @nomeata and @matthewhammer are advocating that we have a Buffer implementation whose type is stable (and thus, not OO-based).

[nomeata]

Same for HashMap please, another very popular module I think :-)

[jzxchiang1]

If HashMap is stable, then wouldn't that basically be a superior version of Trie in almost every way? (except for the fact that Trie is purely functional and can also be shared)

From my limited understanding, it seems like hash tables are just overall more performant than functional hash tries.