LRU cache for ~=

[dsimcha]

== Quote from Andrei Alexandrescu (SeeWebsiteForEmail at erdani.org)'s article
> Making ~= work well for slices does not preclude creating a distinct
> library type.
> Andrei

Yes, I see LRU/MRU/whatever as a kludge to allow convenience without being unsafe
egregiously inefficient.  The fact that it is a kludge is not a criticism because
there is clearly no easy answer, and thus a kludge is genuinely necessary.
However, I think there needs to be a separate array builder type for "heavy duty"
appending operations.  In TDPL I would just say that concatenating slices can be
inefficient and that people should use array builder for heavy duty appending,
length changing, etc., w/o getting into the details.