Extension of java.text.Format for locale-sensitive Bitcoin value formatting & parsing.
Improved `toString()` method; now shows optional decimal groups visually as part of pattern, rather than
as a list of integer values.
Changed one particular identifier to be `volatile`.
Testing awareness of default Locale now sets and tries different Locales.
Parameterize `BtcFormatTest` with every available `Locale`.

Also changed certain tests that examine behavior that is
orthogonal to `Locale` to use the `US` locale rather than the
JVM's default `Locale`.