std/tables [] vs. add()

[cblake]

I retain the duplicate key/multitable/multiset feature in my `lptabz` in the 
[adix](https://github.com/c-blake/adix) package as mentioned [at the end of 
this thread](https://forum.nim-lang.org/t/5917) and also [this 
one](https://forum.nim-lang.org/t/6944). That's the only compatibility support 
of which I am aware.