Reconcile wallet.internal and wallet.loader.LoadedWallet()

[itswisdomagain]

Follow up to #57.

The wallet is considered to be open if wallet.internal != nil. wallet.internal is assigned whenever a wallet is created or opened using wallet.loader, which also sets the created/opened wallet to wallet.loader.LoadedWallet().

If there are scenarios where the wallet is unloaded but wallet.internal is not unset, the impression would be that there is an opened/loaded wallet, when there really isn't.

Even if such scenarios do not come up, it's quite redundant to store the same variable in 2 places when 1 will suffice.

Proposal is to remove the wallet.internal property and introduce a wallet.internal() alias method that return wallet.loader.LoadedWallet().