Add InstanceKey.Value method

Have the InstanceKey implementations handle their own cty.Value
conversions.

addrs/instance_key.go

@@ -18,6 +18,10 @@ import (
 type InstanceKey interface {
 	instanceKeySigil()
 	String() string
+
+	// Value returns the cty.Value of the appropriate type for the InstanceKey
+	// value.
+	Value() cty.Value
 }
 
 // ParseInstanceKey returns the instance key corresponding to the given value,
@@ -56,6 +60,10 @@ func (k IntKey) String() string {
 	return fmt.Sprintf("[%d]", int(k))
 }
 
+func (k IntKey) Value() cty.Value {
+	return cty.NumberIntVal(int64(k))
+}
+
 // StringKey is the InstanceKey representation representing string indices, as
 // used when the "for_each" argument is specified with a map or object type.
 type StringKey string
@@ -69,6 +77,10 @@ func (k StringKey) String() string {
 	return fmt.Sprintf("[%q]", string(k))
 }
 
+func (k StringKey) Value() cty.Value {
+	return cty.StringVal(string(k))
+}
+
 // InstanceKeyLess returns true if the first given instance key i should sort
 // before the second key j, and false otherwise.
 func InstanceKeyLess(i, j InstanceKey) bool {