stage1: add error for slice.len incr beyond bounds
comptime direct slice.len increment dodges bounds checking but
we can emit an error for it, at least in the simple case.
- promote original assert to compile-error
- add test case
closes #7810
f9b85c6e50Michael Dusan committed on 1/30/2021, 9:19:25 AM· committed by Veikka Tuominenparent3d4eeaf