Optimise looking up unused HashMap keys

This optimises lookups of unused keys. When performing a lookup using an
unused key, we would cycle over all buckets in a HashMap. This commit
changes lookups so that they return early the moment they find a missing
bucket.

runtime/src/std/hash_map.inko

@@ -199,6 +199,21 @@ object Table!(K: Hash + Equal, V) {
     let mut pair = @buckets[index]
 
     { pair.key != key }.while_true {
+      # Finding an empty bucket can mean two things:
+      #
+      # 1. The initial desired bucket is not used, meaning our key definitely
+      #    does not exist.
+      # 2. The initial desired bucket is used, and we ran into the next
+      #    available bucket. This means the key also does not exist, because it
+      #    would otherwise use this available bucket.
+      #
+      # This early return ensures we don't iterate over all buckets if we are
+      # certain we won't be able to find the key.
+      pair == Nil
+        .if_true {
+          return
+        }
+
       index = desired_bucket(index + 1)
 
       index == desired