Implement ldexp and use it in ratio_to_f64

[MattX]

This eliminates errors where we attempt a multiplication by 2^-x, but that 2^-x underflows. (#91).

This is based on the code in #94.

The ldexp implementation strategy is mostly to extract the exponent, modify it, then reinsert it. To avoid dealing with subnormal numbers at the bit level, we shift any subnormal numbers into normal range before that operation.

Fixes #91.

[cuviper]

Thanks, and especially the test coverage of ldexp looks great!

bors r+

[bors[bot]]

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