`const_buffer`/`mutable_buffer` constructors aren't usable in constant expressions (pre-C++26)

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The const_buffer and mutable_buffer constructors are marked constexpr, but they cannot actually be evaluated in a constant expression before C++26.

The classes store the pointer as unsigned char const*, so the constructor must do:

p_(static_cast<unsigned char const*>(data))

Casting a void const* to unsigned char const* is not allowed in a constant expression before C++26 (P2738R1).