Regarding Decimal:

  1) The context precision isn't used for formatting.  If you have
     another reason for proposing the optional context argument for
     dec_format(), please open another issue.

  2) There's another problem: The mythical DefaultContext (which
     acts as a template for creating new contexts) affects not only
     new thread-local contexts, but also a new Context()!

     In my opinion this is something we should change: The mechanism
     is fine for thread-local contexts, but Context() should behave
     like a pure function.

  3) The double rounding issues are more tricky than it might seem;
     if we use Decimal for this, perhaps direct support in the module
     would be the cleanest option.