Fix or remove FIXME in std::smallintmap

[catamorphism]

A FIXME in std::smallintmap says: "FIXME: Should not be @; there's a bug somewhere in rustc that requires this to be" (as in: the definition of the smallintmap type is @{mut v: [mut option<T>]} and should be {mut v: [mut option<T>]}). I tried fixing this (and changing insert to take its first argument by pointer), but maybe there's still a bug in rustc, as the fix causes map lookups to fail. So it seems more complicated.

[msullivan]

Presumably it is getting passed by value somewhere or another and so it gets copied? What /should/ the behavior of copying a smallintmap be?

[catamorphism]

Obsolete.