Update to crystal 0.22.0 (#50)

* `WeakRef#target` was renamed to `WeakRef#value`

src/db/pool.cr

@@ -54,7 +54,7 @@ module DB
       # TODO honor candidates while waiting for availables
       # this will allow us to remove `candidates.includes?(resource)`
       candidates.each do |ref|
-        resource = ref.target
+        resource = ref.value
         if resource && is_available?(resource)
           @available.delete resource
           resource.before_checkout
@@ -63,7 +63,7 @@ module DB
       end
 
       resource = checkout
-      {resource, candidates.any? { |ref| ref.target == resource }}
+      {resource, candidates.any? { |ref| ref.value == resource }}
     end
 
     def release(resource : T) : Nil

src/db/pool_prepared_statement.cr

@@ -40,7 +40,7 @@ module DB
     private def clean_connections
       # remove disposed or closed connections
       @connections.each do |ref|
-        conn = ref.target
+        conn = ref.value
         if !conn || conn.closed?
           @connections.delete ref
         end