Update Core Contracts to be compatible with Stable Cadence

[dsainati1]

Problem

https://github.com/onflow/flow-core-contracts/blob/master/contracts/LockedTokens.cdc#L231

The linked code does not behave the way it appears to; the as? in this line does not actually function as a reference operator. In a future version of Cadence this code will be rejected by the typechecker.

Steps to Reproduce

Typecheck this contract on the master version of Cadence.

[joshuahannan]

I've made a PR to address stable cadence changes: #319

[turbolent]

@joshuahannan I guess this is complete and can be closed?

[joshuahannan]

yeah