Done with:
```
git grep -l 'for.*:= 0.*++' \
| xargs sed -i -E 's/for (.*) := 0; \1 < (.*); \1\+\+/for \1 := range \2/'
```
And a few manual fixes due to unused variables. There is something fishy
in BMP rib test. Add a comment about that. This is not equivalent (as
with range, random is evaluated once, while in the original loop, it is
evaluated at each iteration). I believe the intent was to behave like
with range.
validate is only able to validate non-struct types (or recurse inside
struct). So, if we want to use "required" on some of them, we need a
custom type.
Fix#263
