Deserializing CQL collections causes frequent reallocations of Vec

[pkolaczk]

This is an issue I found when testing the performance of vector type, but apparently this applies to all collections.
The code uses itertools to collect fallible items into a Vec<Result<...>>:

                ListlikeIterator::<'frame, T>::deserialize(typ, v)
                    .and_then(|it| it.collect::<Result<_, DeserializationError>>())
                    .map_err(deser_error_replace_rust_name::<Self>)

Unfortunately itertools does not use the size hint in this case and does not reserve proper space in the returned vector.
Instead, it grows the vector as more items are added.

Also it the size hint for ListLikeIterator doesn't look correct anyways (it delegates to raw iterator, which provides the default None hint).

This can be trivially fixed by creating the vector with proper capacity first (we know the number of elements very early on and it is stored in the field of ListLikeIterator) and then adding the items with e.g. extend.

[wprzytula]

@pkolaczk Thanks for pointing this out!