Render HTML-safe JSON strings, especially for use in <script> tags

[allenap]

[Presently there's no issue for this, though #181 might be closest. I use this with Maud in a private codebase of my own, and I thought it could be more widely useful. I also wanted to expose my approach to the light to check if it does actually make sense.]

Reproduced from maud/src/json.rs:

This module provides a [Json] wrapper type which will render its inner value as HTML-safe JSON, and an implementation of [Render] for [serde_json::Value] as a convenience.

This does not follow WHATWG advice for embedding content into <script> tags, nor does it follow the JSON-LD spec's advice.

The WHATWG's advice for <script> elements suggests replacing < characters with \x3C. This works when embedding JSON into a <script> element that is being used for JavaScript because \xNN escapes are allowed in JavaScript, but it does not work when embedding, say, JSON-LD into a <script> element. These \xNN escape sequences are not recognised by JSON parsers, and their use renders the JSON payload invalid.

The JSON-LD spec suggests replacing several characters – <, >, &, and quotes – with HTML entities. It follows that every receiving processor must then reverse this transformation, but this does not seem to happen in practice.

Instead, this module replaces <, >, and & characters in JSON strings (including object keys) with \u003c, \u003e, and \u0026 Unicode escape sequences respectively. This is understood by JSON parsers but is inert as far as HTML <script> tags are concerned, and neutralises misinterpretation of embedded HTML, XML, entities, and other <…> tagged content by HTML processors. Notably, this also prevents HTML comments (i.e. <!-- … -->) from being opened or prematurely closed by JSON within the <script> tag. The JSON content is still entirely valid and able to be parsed as-is, with no pre- or post-processing required; it could be copied and pasted from the source document into a .json file and it would work.

⚠️ Some of the code in here can panic. That's not ideal. I was wondering about having a TryRender trait, say, that would allow for fallible renderers. Alternatively, I think the expect(...) call could remain in the impl Render for serde_json::Value because Value should be almost/entirely guaranteed to be valid JSON. The Json wrapper could be removed; just ask callers to use serde_json::to_value themselves.

---

There are also a few things I fixed in passing, but I can extract those to a separate PR if necessary.