Add convenience functions for conversion to primitive integer types.

[stencillogic]

Consider adding conversion functions to primitive integer types.
Possible signatures (<int_type> is i8, i16, i32, i64, i128, u8, u16,u32, u64, u128):

• BigFloat::try_to_<int_type>(&self, RoundingMode) -> Result<<int_type>, Error>: returns Error if BigFloat's absolute values is larger than the the value <int_type> can hold.
• BigFloat::to_<int_type>_saturating(&self, RoundingMode) -> <int_type>: if the value of BigFloat is larger than the value that <int_type> can hold, clamp the value to the maximum value of <int_type>.

[atrabattoni]

I think this would work in concert will all the floor/ceil/round kind of functions. In my case I need to go back and forth from i128/u128 to f64. I need higher precision floats as intermediate processing values but ultimately i need to go back to primitie.

[atrabattoni]

So the to_<int_type>_saturating would follow how as now works when casting from <float_type> as <int_type>. Looks good to me.