src: use core instead of std in some places, aiming for no_std compat

src/geometry.rs

@@ -1,7 +1,7 @@
-use std::fmt::{Debug, Formatter};
-use std::ops::{Mul, Sub, Add};
+use core::fmt::{Debug, Formatter};
+use core::ops::{Mul, Sub, Add};
 use num::{One, pow, integer::Roots};
-use std::cmp::{min, max};
+use core::cmp::{min, max};
 
 pub trait CoordinateOp: PartialOrd + PartialEq + Sub + Clone + Mul + Copy + One + Add + Eq + Debug where
 Self: Sub<Output=Self> {
@@ -21,7 +21,7 @@ pub struct Coordinates<S: CoordinateSpace> {
 }
 
 impl<S: CoordinateSpace> Debug for Coordinates<S> where S::Data: Debug {
-    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+    fn fmt(&self, f: &mut Formatter<'_>) -> core::fmt::Result {
         f.debug_tuple("@")
             .field(&self.x)
             .field(&self.y)