uint256: Add basic usage example.

This adds an example of calculating the result of dividing a max
unsigned 256-bit integer by a max unsigned 128-bit integer and
outputting that result in hex with leading zeros.

This is part of a series of commits to fully implement the uint256
package.

internal/staging/primitives/uint256/example_test.go

@@ -0,0 +1,24 @@
+// Copyright (c) 2021 The Decred developers
+// Use of this source code is governed by an ISC
+// license that can be found in the LICENSE file.
+
+package uint256_test
+
+import (
+	"fmt"
+
+	"github.com/decred/dcrd/internal/staging/primitives/uint256"
+)
+
+// This example demonstrates calculating the result of dividing a max unsigned
+// 256-bit integer by a max unsigned 128-bit integer and outputting that result
+// in hex with leading zeros.
+func Example_basicUsage() {
+	// Calculate maxUint256 / maxUint128 and output it in hex with leading zeros.
+	maxUint128 := new(uint256.Uint256).SetUint64(1).Lsh(128).SubUint64(1)
+	result := new(uint256.Uint256).Not().Div(maxUint128)
+	fmt.Printf("result: %064x\n", result)
+
+	// Output:
+	// result: 0000000000000000000000000000000100000000000000000000000000000001
+}