Implement floor, ceil and round

[frectonz]

Closes #155

I am not sure if converting BigRats into f64s and then doing the operations on them is the right way to implement this but this is what i was able to come up with. I am happy to modify the code based on any suggestions on how to do this better.

pub(crate) fn floor<I: Interrupt>(self, int: &I) -> FResult<Self> {
  let float = self.into_f64(int)?.floor();
  Self::from_f64(float, int)
}

pub(crate) fn ceil<I: Interrupt>(self, int: &I) -> FResult<Self> {
  let float = self.into_f64(int)?.ceil();
  Self::from_f64(float, int)
}

pub(crate) fn round<I: Interrupt>(self, int: &I) -> FResult<Self> {
  let float = self.into_f64(int)?.round();
  Self::from_f64(float, int)
}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fb02762) 83.20% compared to head (eda5977) 83.30%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #269      +/-   ##
==========================================
+ Coverage   83.20%   83.30%   +0.10%     
==========================================
  Files          52       52              
  Lines       14532    14623      +91     
==========================================
+ Hits        12091    12182      +91     
  Misses       2441     2441              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[printfn]

Looks good, thanks!