Upgrade to Go v1.18 and add support for generics

.travis.yml

@@ -1,9 +1,7 @@
 language: go
 
 go:
-  - 1.16
-  - 1.15
-  - 1.14
+  - 1.18
 
 env:
   - GO111MODULE=on

cache.go

@@ -3,26 +3,26 @@ package dataloader
 import "context"
 
 // The Cache interface. If a custom cache is provided, it must implement this interface.
-type Cache interface {
-	Get(context.Context, Key) (Thunk, bool)
-	Set(context.Context, Key, Thunk)
-	Delete(context.Context, Key) bool
+type Cache[K comparable, V any] interface {
+	Get(context.Context, K) (Thunk[V], bool)
+	Set(context.Context, K, Thunk[V])
+	Delete(context.Context, K) bool
 	Clear()
 }
 
 // NoCache implements Cache interface where all methods are noops.
 // This is useful for when you don't want to cache items but still
 // want to use a data loader
-type NoCache struct{}
+type NoCache[K comparable, V any] struct{}
 
 // Get is a NOOP
-func (c *NoCache) Get(context.Context, Key) (Thunk, bool) { return nil, false }
+func (c *NoCache[K, V]) Get(context.Context, K) (Thunk[V], bool) { return nil, false }
 
 // Set is a NOOP
-func (c *NoCache) Set(context.Context, Key, Thunk) { return }
+func (c *NoCache[K, V]) Set(context.Context, K, Thunk[V]) { return }
 
 // Delete is a NOOP
-func (c *NoCache) Delete(context.Context, Key) bool { return false }
+func (c *NoCache[K, V]) Delete(context.Context, K) bool { return false }
 
 // Clear is a NOOP
-func (c *NoCache) Clear() { return }
+func (c *NoCache[K, V]) Clear() { return }