Image resource refactor

This commit pulls most of the image related logic into its own package, to make it easier to reason about and extend.

This is also a rewrite of the transformation logic used in Hugo Pipes, mostly to allow constructs like the one below:

    {{ ($myimg | fingerprint ).Width }}

Fixes #5903
Fixes #6234
Fixes #6266

resources/internal/key_test.go

@@ -0,0 +1,36 @@
+// Copyright 2019 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package internal
+
+import (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+type testStruct struct {
+	Name string
+	V1   int64
+	V2   int32
+	V3   int
+	V4   uint64
+}
+
+func TestResourceTransformationKey(t *testing.T) {
+	// We really need this key to be portable across OSes.
+	key := NewResourceTransformationKey("testing",
+		testStruct{Name: "test", V1: int64(10), V2: int32(20), V3: 30, V4: uint64(40)})
+	c := qt.New(t)
+	c.Assert("testing_518996646957295636", qt.Equals, key.Value())
+}