Make seq immutable in it's own iterator scope

[Stuffe]

Summary

Make seqs immutable in their own iterator scope.

Description

Right now calling for example .add() on a seq inside its own iterator scope will always result in a run time error. It is as sure to crash as assert(false). Yet sometimes this can go unnoticed in a branch. And right now, fixing this can be cumbersome since the error message also does not contain the line number.

Here is an example:
https://play.nim-lang.org/#ix=3uLg

Note that the index and value iterated over are already immutable inside an iterator scope, but what I ask is for the seq variable itself to also be immutable.

[Araq]

Seems to be handled well once we enable more borrow checking...