[Altren]

Fixed-size arrays currently carry a redundant length prefix in the Cereal, YAS and Boost.Serialization adapters, even though the extent is already part of the C++ type. Serialize std::array and C arrays without that prefix, including nested and empty fixed-size arrays. This saves one length field per fixed array (8 bytes in the 64-bit binary configurations).

Add an optional read_fixed_array adapter API and matching writer operations. Fixed and dynamic archive reads share the element-reading loop; fixed reads take their extent from the type, while dynamic reads consume their existing length prefix. Cereal and YAS input array handles remain pointer-only. Map keys, variant tags and element parsing through processors are preserved, and formats without the new API retain their existing array representation.

Compatibility: this changes the fixed-array wire format in these three adapters. Existing archives need to be read with the previous version and rewritten. Readers and writers must agree on the array type and extent; without the prefix, an extent mismatch cannot be reliably detected.

This is extremely useful for data storages like std::vector<Eigen::Vector2d> and custom reflectors, that represent such vectors as std::array<double, N>. Reducing stored size by 33% and improving both read and write performance by 30% for Cereal and Boost and 10% for YAS

[liuzicheng1987]

@Altren thanks!