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/stack_layout.rs

@@ -19,7 +19,7 @@ pub fn layout_stack(frame: &mut StackSlots, alignment: StackSize) -> Result<Stac
     // Each object and the whole stack frame must fit in 2 GB such that any relative offset within
     // the frame fits in a `StackOffset`.
     let max_size = StackOffset::max_value() as StackSize;
-    assert!(alignment.is_power_of_two() && alignment <= max_size);
+    debug_assert!(alignment.is_power_of_two() && alignment <= max_size);
 
     // We assume a stack that grows toward lower addresses as implemented by modern ISAs. The
     // stack layout from high to low addresses will be:
@@ -70,7 +70,7 @@ pub fn layout_stack(frame: &mut StackSlots, alignment: StackSize) -> Result<Stac
     // The offset is negative, growing downwards.
     // Start with the smallest alignments for better packing.
     let mut offset = incoming_min;
-    assert!(min_align.is_power_of_two());
+    debug_assert!(min_align.is_power_of_two());
     while min_align <= alignment {
         for ss in frame.keys() {
             let slot = frame[ss].clone();