Definitely 'ipairs' and the metamethod on 'pairs'. The former is usually
better done using numeric 'for' and can be trivially implemented in Lua
if wanted. The latter can be done by replacing 'pairs' or by writing
another iterator with a name reflecting what it does.
Generally I'd remove everything from the standard libraries that does
not justify implementation in 'C' either because it cannot be done in
Lua or because serious performance gains can be had. Then I'd have
standard Lua implementations for the rest.