Use fixed-width integer types

Certain algorithms (like decoupled look-back) strongly depend on the type sizes while still using standard integer types like `unsigned long long int`. We should use fixed-width integer types like `std::uint64_t` in those places instead.

The issue names no files or tests. Start by locating the decoupled look-back algorithm and other places that use standard integer types, then determine which type sizes are relied upon. Done means those places use fixed-width types such as std::uint64_t without changing the algorithms' intended behavior.

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