wereAddressesSpentFrom private and internal use

[Cylix]

From #304 (iota.lib.js changes):

wereAddressesSpentFrom() is no longer exported - We don't want devs to build solutions around a method which might be infeasible in IoT environment. It's still being used under the hood in the following methods, until local store solution is implemented:
* getNewAddress(): https://github.com/iotaledger/iota.lib.js/blob/next/packages/core/src/createGetNewAddress.ts#L23
* getAccountData(): https://github.com/iotaledger/iota.lib.js/blob/next/packages/core/src/createGetAccountData.ts#L168 & https://github.com/iotaledger/iota.lib.js/blob/next/packages/core/src/createGetAccountData.ts#L179
* getInputs(): need to be added - issue TBD

Two things here:

• For removing the export, we can move it as a private function and check with iota.lib.py and iota.lib.java about their position regarding this.
• We can look about how this function is used internally to use it as well if we don't