Implement slashing of non-liquid delegated stake

[DrZoltanFazekas]

While slashed deposits are automatically reflected in the LST price of liquid delegated staking, the non-liquid variant needs to implement slashing explicitly. It requires the following changes:

1. Store the slashing events consisting of the index of the staking event after which they happened and the slashed fraction.
2. Adjust the staked amount when a user stakes or unstakes a value from the current amount + value to amount * product_of_slashed_fractions + value where product_of_slashed_fractions = (1 - slashings[i].fraction) * (1 - slashings[i+1].fraction) * ... * (1 - slashings[slashings.length - 1].fraction) for the smallest i such that slashings[i].index >= stakingIndices[user][stakingIndices[user].length - 1].

Note that the rewards are not directly impacted by slashing.