Make BitArray a little constexpr-friendly

[abacabadabacaba]

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[github-actions]

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[DanShaders]

Removes operator< for n % 8 != 0 case and the explicit defaulted copy constructor. Don't think this matters much though.

[github-actions]

• crypto/common/bitstring.h:513: operator=(ConstBitPtr) now assigns through BitArray(from). For non-byte-aligned n, that constructor leaves the unused bits in the last byte uninitialized, then copies the whole std::array into *this. Previously assignment copied only n bits into existing storage, so a zeroed tail stayed zero. This can make as_slice()/as_array() expose nondeterministic tail bits after BitArray<15> x = BitArray<15>::zero(); x = ptr;. Keep the direct bits_memcpy assignment path or zero-initialize the temporary storage first.

• crypto/common/bitstring.h:574: non-byte-aligned BitArrays lost relational ordering. The old operator< worked for every n, but the new <=> is only declared for n % 8 == 0, and compare() is not used for rewritten relational operators. Existing or downstream code using BitArray<15>/BitArray<267> in ordered comparisons or ordered containers will no longer have the same API.