Procedurally generate packing libraries

[Amxx]

Relevant proposals:

• Procedurally generate packing libraries #5051
• Extended packing and extracting library for value types #5056
• https://github.com/Amxx/openzeppelin-contracts/tree/feature/extract

Goal:

• Address the need of the users
  • need to idendify usecases
    • splitting ERC-4337 values (packed gas data, meta-nonce)
• provide all the necessary tools in a single library (if possible)

Possible features:

• packing/splitting an uint256/bytes32 into chunks of various sizes (128-128, 64-192, 192-64, 208-48, 32-224, 224-32, ...) ?
• packing/splitting other types (smaller) into chunks of various sizes ?
• packing/splitting an array of chunks of similar sizes into a bigger object (put Uint64x4 in a uint256)
• (low level) extract arbitrary bytesXx/uintXx from a bigger object, given an offset

Question:

• what are the type we pack/extract ? bytesXx (aligned left) or uintXxx (aligned right) ?
• what do we want to pack them into ? bytesXx (aligned left) or uintXxx (aligned right) ?
• do we want to split/pack more then two value at once, or do we consider composition for more complex types:
  For example: spliting a uint256 into two uint128, and then spliting one of the uint128 into two uint64 allows us to access any uint64 in the initial uint256