build: power_scalar_base behind no_alloc feature

Co-authored-by: Shahar "Dawn" Or <[email protected]>

Author: warren2k

src/size_hint.rs

@@ -3,6 +3,7 @@
 
 use std::usize;
 use std::cmp;
+#[cfg(feature = "use_alloc")]
 use std::u32;
 
 /// `SizeHint` is the return type of `Iterator::size_hint()`.
@@ -76,6 +77,7 @@ pub fn mul_scalar(sh: SizeHint, x: usize) -> SizeHint {
 }
 
 /// Raise `base` correctly by a `SizeHint` exponent.
+#[cfg(feature = "use_alloc")]
 #[inline]
 pub fn pow_scalar_base(base: usize, exp: SizeHint) -> SizeHint {
     let exp_low = cmp::min(exp.0, u32::MAX as usize) as u32;