Function cautious_capacity 

pub(crate) fn cautious_capacity<K, V>(hint: Option<usize>) -> usize

Limit our preallocated capacity from a deserializer size_hint().

We do account for the Bucket overhead from its saved hash field, but we don’t count the RawTable allocation or the fact that its raw capacity will be rounded up to a power of two. The “max” is an arbitrary choice anyway, not something that needs precise adherence.

This is based on the internal serde::de::size_hint::cautious(hint) function.