Add single-threaded implementation of Store::analyze_content

[jpeddicord]

To solve WASM (#1), threads probably have to be dropped for that target.

analyze_content needs a twin that does the same thing, just without Rayon. I'm not sure if there's a lot of code re-use possible here, but the closure inside fold can probably be factored out. Not too difficult, just needs some tinkering.

[jpeddicord]

This may not be an issue; emscripten might stub this out anyway.

[jpeddicord]

Done in 0ab9eef