Allocate space for hashing functions workspaces

[BowTiedWoo]

This PR refactors the memory management for hashing functions (sha256, sha512, and hash160) by introducing a dedicated workspace for each function.

Instead of allocating memory every time a hash is computed, each function now allocates its workspace once during initialization. The memory location for each workspace is stored and reused, improving performance and reducing allocation overhead.

Closes: #645

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