Recommend const functions instead of static variables

[clarfonthey]

Now that const fn invocation is stabilised for the standard library at least, it makes sense to suggest replacing static variables with their const equivalents, namely:

• ATOMIC_X_INIT => AtomicX::new(0)
• ONCE_INIT => Once::new()
• uX/iX::MAX/MIN => uX/iX::max/min_value()