Reading floats with large exponents no longer takes too much time.

Reading floats with large exponents no longer takes too much time.

Reading 1.0s1000000000000000 will attempt to construct a very large
bignum, which takes a considerable amount of time just to report in
the end that it cannot be represented as a float. Truncate the
exponent to manageable size before raising it.

Fixes lp#309070. Thanks to Paul Khuong for the help.