This is required to declassify pointers to constant memory. Declassify should
never modify its argument.
This is a partial backport of secp256k1 [[https://github.com/bitcoin-core/secp256k1/pull/558 | PR558]] : https://github.com/bitcoin-core/secp256k1/pull/558/commits/3e08b02e2a78f2a1fc457efab665db8ab8085373