Use object URLs for inlined linked modules

[daxpedda]

Currently link_to!() without --split-linked-modules would build a encoded URL and return it as a String when accessed.
This makes link_to!() quite pointless in this case as its not much better then using include_str!() in Rust.

This PR changes this case to use object URLs instead. The resulting URL is also cached on the Rust side with the help of once_cell (which was a dependency already), so we don't keep creating new object URLs over and over, which would be a memory leak.

A fallback is built-in for service-workers, where the API isn't present.

So this change is a trade-off, the URL is now permanently stored in Wasm memory once called, but the URL is always guaranteed to be quite short, except in the case of service workers. In contrast, when not using --split-linked-modules, the whole file was transferred into Wasm memory.

Follow-up to #3069.