ICU4x number formatting

[JasperDeSutter]

Adds basic decimal formatting for FluentNumber.as_string(). Many features are still missing like currency symbols and percentage formatting. #269

Since data providers need to be managed explicitly, it needs to be handed to the memoizer to be able to construct a formatter. This required a new argument + associated type on Memoizable.

Something to figure out is where the data comes from. I've went with a simple FluentBundle.set_number_format_provider, which seems most flexible to me. I've added a test that uses icu_testdata for this, but other providers should work as well. https://icu4x.unicode.org/doc/icu/#data-management