Copy and truncate rational results exactly

Bug: 26966394
Bug: 26175989

Provide a separate BoundedRational toString() function to handle
generation of decimal strings from rational results.  This ensures
that rational result are always displayed as correctly truncated.
Use this facility to generate an exact representation when copying
a rational result, and when doing so is appropriate.

Includes some minor related changes to the same code:

- Refuse to copy a result that is currently being evaluated.
- Increase the bound on rational size again, to make all of this
  more effective.
- Remove one line of dead code, and fix some comments, etc.

Change-Id: I5f72d5e47849ceeb2f1b6be870eb2a64edd5d508