Add copysign() export
Add isfinite, isinf and exponent functions

exponent has a '- 1' at the end of it to make it work as the
Base.exponent from libm.
Add isnan function
Add conversion to Float32
Throw DomainError() for the square root of a negative MPFRFloat
Add a tentative way to globally change the used precision

The functions `get_default_precision` and `set_default_precision`
get and set, respectively, the globally used precision.
Add a `with_precision` function which executs a block in a different precision
Add `integer_valued`
Add `nextfloat` and `prevfloat` methods
Add checks to verify the asked precision is above the minimum
Add new comparison functions to handle correctly with NaNs