strbasics.setSlice that takes BackwardsIndex

[juancarlospaco]

I have precalc reverse index of a string that I need to cut,
and I want to use setSlice on a string, but it do not take BackwardsIndex, I know is possible to cast it to int,
but would be great an overload to use BackwardsIndex directly, should be doable.

If you have to call aditional functions to use setSlice kinda defeats the purpose of having fast performance.

setSlice(s, slice = ^9 .. ^2 )

• https://nim-lang.github.io/Nim/strbasics.html#setSlice%2Cstring%2CSlice%5Bint%5D

/cc @xflywind