Mark Packed64x4.sol as Unsafe or Deprecated

[tamtamchik]

While covering libs with test in the core repo, I stumbled upon some wired behavior of the contracts/0.4.24/lib/Packed64x4.sol's set function.

Passing an invalid n (n > 3) in the set function can lead to unexpected behavior.

• In HardHat tests calling set(4,5) leads to rewriting value for the first spot, so calling get(0) returns 5 instead of the original value.
• In Foundry tests it just errors with: Error occurred: invalid opcode

There are no invalid usages inside the current codebase, so no worries 😉

But it is better not to use this library in new contracts as is.
Or at least add a require(n < 4, "OFFSET_OUT_OF_RANGE"); check to the set function.