NumberStyle: add rudimentary support for exponentials.

Also, minor updates to Conversions to make it work there.

NB: This is clearly not properly factored, yet! This is the first step
to get feedback and improvements from @mintsoft.

- Make number style stuff aware of how to deal with case-insensitive
  expoentials.
- Produce consistent output of such strings.

The display format was chosen to be consistent with the output for
Calculator.  Also, the parsing is not quite correct as you can't have a
mantissa in the exponent, but this won't catch that.

Should we also do "for_display" on the factors for consistency?

Any and all feedback or improvements are appreciated.

This also breaks the Calculator tests, but I will worry about fixing
those once everything stabilizes.