Implement Big UInt Square for modexp

[ilitteri]

Context: modexp.yul

Description:

/// @notice Performs the big unsigned integer square of big unsigned integers with an arbitrary amount of limbs.
/// @dev The product is stored from `productPtr` to `productPtr + (WORD_SIZE * nLimbs)`.
bigUIntSqr(numberPtr, nLimbs, productPtr)

numberPtr: The pointer to the MSB of the big unsigned integer to square.
nLimbs: The number of limbs needed to represent the number to square.
productPtr: The pointer to where you want the product to be stored.

Recommendation:

Follow this implementation.