Added staking events

[Shloyem]

Added Staked and StakeWithdraw event and emitting them accordingly.

Deliberating:

• If its a good idea to add an events contract, so Staked and StakeWithdraw can be shared and reused for SimpleStakingV2

and if not, whether to add a event to:

• transfer function: It's parent/super transfer emits a transfer event
• withdrawRewards