numberToRangedRational: fix edge cases for exp ≈ (maxBound :: Int)

Currently a negative exponent less than minBound :: Int results in Infinity, which is very surprising and obviously wrong.

λ> read "1e-9223372036854775808" :: Double                          
0.0                                                                 
λ> read "1e-9223372036854775809" :: Double                          
Infinity

There is a further edge case where the exponent can overflow when increased by the number of tens places in the integer part, or underflow when decreased by the number of leading zeros in the fractional part if the integer part is zero:

λ> read "10e9223372036854775807" :: Double                          
0.0                                                                 
λ> read "0.01e-9223372036854775808" :: Double                       
Infinity

To resolve both of these issues, perform all arithmetic and comparisons involving the exponent in type Integer. This approach also eliminates the need to explicitly check the exponent against maxBound :: Int and minBound :: Int, because the allowed range of the exponent (i.e. the result of floatRange for the target floating point type) is certainly within those bounds.

---

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