Make `.Ok()` on Arrays Faster

The generated code for `.Ok()` on an array is basically a loop, like:

```
bool Ok() const {
for (int i = 0; i < ElementCount(); ++i) {
if (!(*this)[i].Ok()) return false;
}
return true;
}
```

When array elements are simple types, such as `UInt` with no constraints, `.Ok()` on each element devolves to a bounds check; unfortunately, no C++ compiler appears to implement an optimization that hoists the bounds checks out of the loop, so the array `.Ok()` method ends up repeating a huge amount of busywork.

To remedy this, the C++ back end could emit static functions on views like `bool OkIsEquivalentToIsComplete()` and `bool IsCompleteIsAStaticSizeCheck()`, then the array's `.Ok()` could be something like:

```
bool Ok() const {
if (decltype((*this)[0])::OkIsEquivalentToIsComplete() &&
decltype((*this)[0])::IsCompleteIsAStaticSizeCheck()) {
return (*this)[0].Ok();
} else {
for (int i = 0; i < ElementCount(); ++i) {
if (!(*this)[i].Ok()) return false;
}
return true;
}
}
```

In C++17, the outer `if` can become `if constexpr`.