Replace assert! with debug_assert! in production code paths.

This allows the assertions to be disabled in release builds, so that
the code is faster and smaller, at the expense of not performing the
checks. Assertions can be re-enabled in release builds with the
debug-assertions flag in Cargo.toml, as the top-level Cargo.toml
file does.

lib/cretonne/src/bitset.rs

@@ -36,8 +36,8 @@ where
 
     /// Check if this BitSet contains the number num
     pub fn contains(&self, num: u8) -> bool {
-        assert!((num as usize) < Self::bits());
-        assert!((num as usize) < Self::max_bits());
+        debug_assert!((num as usize) < Self::bits());
+        debug_assert!((num as usize) < Self::max_bits());
         self.0.into() & (1 << num) != 0
     }
 
@@ -62,8 +62,8 @@ where
 
     /// Construct a BitSet with the half-open range [lo,hi) filled in
     pub fn from_range(lo: u8, hi: u8) -> Self {
-        assert!(lo <= hi);
-        assert!((hi as usize) <= Self::bits());
+        debug_assert!(lo <= hi);
+        debug_assert!((hi as usize) <= Self::bits());
         let one: T = T::from(1);
         // I can't just do (one << hi) - one here as the shift may overflow
         let hi_rng = if hi >= 1 {