#736: Fix format overflow when significand (after dividing down to 19 digits) is 2^63 or greater
Test cases added: large fixed-point number (20 digits for brevity), tiny floating-point (only works up to 18 fixed digits)
Fix Format() overflow on very small numbers, trying to scale them by excessively large powers of 10.
pow( 10.0, -ExpValue ) overflows a double if -ExpValue exceeds 308.
Instead, multiply by pow( 5.0, -ExpValue ) and pow( 2.0, -ExpValue ).
The former is safe as long as -Expvalue <= 441.
Test cases added.