[Foundation] Data: Hash the entire contents, not just an arbitrary subset

[lorentey]

This was cherry picked from commit b711ed9 in #23832. Data already implements hash(into:), so arguably this change doesn't belong in that PR anyway.

This is one case where the existing hashing implementation deliberately fails to feed all relevant information into the hasher. This makes it trivial to induce an arbitrary number of collisions, despite all the effort that went into implementing a high-quality universal hash function.

This seems like a particularly bad idea for Data, which is the standard type for safely exchanging byte buffers across module boundaries. It is reasonable to expect Data to implement the same high quality hashing as Array does; silently doing otherwise is not a good idea.

We previously discussed this issue in #21754.

[shahmishal]

Please update the base branch to main by Oct 5th otherwise the pull request will be closed automatically.

• How to change the base branch: (Link)
• More detail about the branch update: (Link)

[tbkka]

Is this still relevant?