Add Freeze/Thaw events and tests

[sea212]

This PR adds a Freeze and Thaw event. The respective event is emitted whenever some balance is frozen or thawed. This is a continuation of #12287. I decided to split PRs to maintain transparency and to not invalidate the previous audit, see #12287 (comment) for more information.

The logic remained the same as in #12287, however two new events Freeze and Thaw were added that cover the case that update_freezes is invoked instead of the deprecated update_locks. The tests are also equal, except that the calls were adjusted to the freeze case. Those are the test scenarios:

Freezes = []        --> [10]      // emits Frozen(..., amount=10)
Freezes = [10]      --> [15]      // emits Frozen(..., amount=5)
Freezes = [15]      --> [15, 20]  // emits Frozen(..., amount=5)
Freezes = [15, 20]  --> [17, 20]  // emits nothing
Freezes = [17, 20]  --> [17, 15]  // emits Thawed(..., amount=3)
Freezes = [17, 15]  --> [15]      // emits Thawed(..., amount=2)
Freezes = [15]      --> []        // emits Thawed(..., amount=15)

I decided to follow the approach of adding new events for functionality added in the Fungible implementation. Now a Lock and Unlock event exists for calls into the deprecated update_locks, whereas Freeze and Thaw also exist to cover calls into the new update_freezey function. Should we move forward with that approach?

[jsidorenko]

Could you pls sync your branch with master?

[ggwpez]

bot rebase

[paritytech-processbot]

Rebased

[Polkadot-Forum]

This pull request has been mentioned on Polkadot Forum. There might be relevant details there:

https://forum.polkadot.network/t/april-updates-for-substrate-and-polkadot-devs/2764/1