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types: serialize: introduce new helpers for writing values and adjust…
… interfaces Currently, `SerializeRow` and `SerializeCql` traits are just given a mutable reference to a Vec<u8> and asked to append their CQL representation to the end. While simple, there are some issues with the interface: - The serialize method has access to the serialized representation of the values that were appended before it. It's not necessary for a correct implementation to have access to it. - Implementors technically can append any byte sequence to the end, but actually are expected to produce a CQL [value] containing the serialized value. While the `SerializeRow` and `SerializeCql` traits are not generally meant to be manually implemented by the users, we can make the interface easier to use and harder to misuse by making it append-only, restricting what the users are allowed to append and requiring the users to append anything by using a dash of type-level magic. Introduce `RowWriter` and `CellWriter` traits which satisfy the above wishes and constraints, and pass them instead of Vec<u8> in `SerializeRow` and `SerializeCql`. The new traits have two implementations - a Vec<u8> backed one that actually appends the bytes given to it, and a usize-backed one which just measures the length of the output without writing anything. Passing the latter before doing the actual serialization will allow to preallocate the right amount of bytes and then serialize without reallocations. It should be measured whether the reallocation cost always outweighs the calculation cost before implementing this optimization.
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