Speed up nominator state checks in staking pallet (#2153)

Should help https://github.com/paritytech/polkadot-sdk/issues/234.
Related to https://github.com/paritytech/polkadot-sdk/issues/2020 and
https://github.com/paritytech/polkadot-sdk/issues/2108.

Refactors and improves running time for try runtime checks for staking
pallet.

Tested on westend on my M2 pro: running time drops from 90 seconds to 7
seconds.