Nim 1.6.0 RC1

[haxscramper]

Can we do the 1.6 release using old nimble before all of this is sorted out? 
The change is too breaking, so maybe it is a good idea to first figure out how 
to deal with it and then update nimble in one of the patch updates? People who 
want to try out latest nimble can always do `nimble install nimble` and test 
things as they want.