Address FIXME comments from previous cleanup CL

Bug: 22047258

Add code to actively prevent ...99999 from getting turned back into
...00000 .  This may have been imposssible anyway, but I couldn't
prove it.

Add tests for some Evaluator static functions, including the new
unflipZeroes().

Fix a minor bug in exponentEnd() discovered in the process.
The logic for limiting exponent length was wrong.

Remove some no longer needed code to force a reevaluation.

Generalize getPadding() function to repeat() and make it static.
Remove leadingZeroes() which is no longer used.

Make getMsdIndex() return the correct result even if it has changed
due to reevaluation.

Change-Id: Id457d8327ce0bc184d72ef78ddd3dbdaab7c6c78