Stabilized toString output for MoneyContext and MoneyNumeric

[garyKeorkunian]

• Added custom toString methods to MoneyContext and MoneyNumeric to stabilize their output. This should prevent thrash in the README file on each run of tut.

No tests should be required for these methods, as they are primarily used in the REPL and production application code should generally not depend on these values.

[cquiroz]

This looks good, though I'm not sure about the assumption that it wouldn't impact application code. Often code relies on toString for better or worse. Perhaps we should add a note to the Readme/Release notes?

[garyKeorkunian]

Yes, I suppose some apps could be using in logs or other ways I've not anticipated. Once it's merged, I'll add a migration note to the release history.

[derekmorr]

I wonder if we could just map over currencies to get their symbols and make a SortedSet from that. And do the same for rates. That would avoid needing to create Ordering instances for Currency and CurrencyExchangeRate.

Something like:

val cSet = currencies.map { _.code }.sorted.mkString(',')

[garyKeorkunian]

Yes, much cleaner. I needed to throw a toSeq call in the pipeline to provide sortability without the Ordering implicit. I'll have a new push soon.

[garyKeorkunian]

The SortedSet was overkill for the internal implementation, for sure.