Rely on strict increasing order in checkpoints lookups

[0xVolosnikov]

Implements what is mentioned in :
#4540 (comment)

I'm very interested in why the library relies on non-strictly increasing order everywhere, like here:

openzeppelin-contracts/contracts/utils/structs/Checkpoints.sol

Line 164 in b2e7bab

if (_unsafeAccess(self, mid)._key > key) {

Based on the descriptions of the functions, it looks like they were created with the expectation that there may be elements with the same keys:

openzeppelin-contracts/contracts/utils/structs/Checkpoints.sol

Line 151 in b2e7bab

* @dev Return the index of the last (most recent) checkpoint with key lower or equal than the search key, or `high` if there is none.

At the same time, the insertion implementation does not allow adding multiple elements with the same key. This, of course, can be done manually by forcibly adding an element to the array. But you can also add an incorrect element (with a decreasing key) in such way. So it doesn't look like there should be multiple elements with the same key in an array in normal use of the library.

If the array is strictly ordered, then it is strange that the search does not handle the case when it hits the desired key (the case of equality). In the worst case, if the array is N long and the target is exactly in the middle, the current implementation will do something like log2(n) - 1 excessive reads of cold storage slots (~2100 gas each).

So with simple example like (keys) [2, 3, 5, 7, 11, 12, 13, 14] i tried two implementations of lowerLookup: current and with separate handling of equality. Gas estimations for lowerLookup(11) (including 21000 “base fee”):
current version: 31712
modified version: 26762

I'm interested if there's a reason not to make such changes.

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