Rename "Stackmap" to "StackMap"

And "stackmap" to "stack_map".

This commit is purely mechanical.

cranelift/codegen/meta/src/shared/settings.rs

@@ -157,7 +157,7 @@ pub(crate) fn define() -> SettingGroup {
         r#"
             Enable safepoint instruction insertions.
 
-            This will allow the emit_stackmaps() function to insert the safepoint
+            This will allow the emit_stack_maps() function to insert the safepoint
             instruction on top of calls and interrupt traps in order to display the
             live reference values at that point in the program.
             "#,