Merge #1017

1017: Add a 'clear' method to map. r=nlewycky a=nlewycky

# Description
Motivated by work on the `nlewycky/inkwell-lifetimes` branch. No callers yet on master, but we use it on the branch and I'd like to merge in pieces.


Co-authored-by: Nick Lewycky <nick@wasmer.io>

lib/runtime-core/src/structures/map.rs

@@ -37,6 +37,11 @@ where
         }
     }
 
+    /// Clears the map. Keeps the allocated memory for future use.
+    pub fn clear(&mut self) {
+        self.elems.clear();
+    }
+
     /// Returns the size of this map.
     pub fn len(&self) -> usize {
         self.elems.len()