Fix getShortString(), tune evaluation heuristics.

Bug: 21474616

Rewrite getShortString() to also look at the least significant digit
information when available, and try to mimic the display formatting
code wherever appropriate.  As a result, when the user hits "=",
followed by "+" transitions are now more frequently smooth.

Revise the evaluation heuristics so the we are more aggressive
with the initial evaluation precision, and try harder to discover
the leading digit in a near-zero number.  Some of this is necessary
to keep getShortString() happy.  This version should also now
guarantee that we are never worse than double precision floating
pointing in displaying very small nonzero numbers.  If we display
a number as zero, the old calculator would have, too.  (And now
you can scroll to see whether it really is.)

Up the BoundedRational bit limit to improve the chances of identifying
exact results.

In general, the incremental computation cost for operating on
larger BigIntegers appears relatively low, so it makes sense to
trade longer computations for fewer calls.

Change-Id: I33066845b832753c109fcaf27f883b48e7e119d2