Simplify page tree logic

This is preparation for #6041.

For historic reasons, the code for bulding the section tree and the taxonomies were very much separate.

This works, but makes it hard to extend, maintain, and possibly not so fast as it could be.

This simplification also introduces 3 slightly breaking changes, which I suspect most people will be pleased about. See referenced issues:

This commit also switches the radix tree dependency to a mutable implementation: github.com/armon/go-radix.

Fixes #6154
Fixes #6153
Fixes #6152

common/maps/maps_get.go

@@ -0,0 +1,31 @@
+// 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 maps
+
+import (
+	"github.com/spf13/cast"
+)
+
+// GetString tries to get a value with key from map m and convert it to a string.
+// It will return an empty string if not found or if it cannot be convertd to a string.
+func GetString(m map[string]interface{}, key string) string {
+	if m == nil {
+		return ""
+	}
+	v, found := m[key]
+	if !found {
+		return ""
+	}
+	return cast.ToString(v)
+}