properFraction is currently undefined

[ekmett]

mpfr_get_z_2exp should be able to get the significand as an Integer, and with a little bit of multiplication by the 2^exponent we could solve this.

[claudeha]

properFraction is currently defined using modf, but I think it needs a test suite to be sure whether it is correct w.r.t. Haskell2010 expectations on the result.