Can I quick get table last key value use like at(idx) function?

[cblake]

FWIW, this should already work in [adix](https://github.com/c-blake/adix) in 
compact-insertion-ordered mode 
[lptabz](https://github.com/c-blake/adix/blob/master/adix/lptabz.nim#L708). The 
data will not be very predictable if you start doing any deletes, but 
@Angluca's use case sounds very static.