Issue 3652 - Allow explicit and implicit casting of dynamic array slices of known size to static array

https://issues.dlang.org/show_bug.cgi?id=3652

Even if a poor D compiler does not support any const-folding, `arr[1 .. 3]` should be convertible to `T[2]`. This PR will add the minimum requirement for the feature.
Ah small typo, converted should be with one "n" :)