SpiderMonkey uses the `encoding_c` Rust crate and comes bundled with it. This lead to either duplication or linking errors when using our own crate re-exporting `encoding_c`.

Instead, with this PR we just bundle a C header file for interacting with the crate's functionality, and rely on SpiderMonkey's version.

…eady been consumed

`HostString` converts to `string_view`, so this makes `decode` easy to use from more places.

…t API abstraction and various other fixes

…ring wizening

The idea is that with this, builtins handling runtime events can dynamically load and execute a top-level script as needed.

This functionality isn't actually used in this commit, but will be in another commit changing how incoming HTTP requests are handled.

Gets back to passing all current tests, and most of the WPT suite that Fastly's JS Compute runtime passes.

fix: release build

This changes the implementation of the handle abstraction to make it more robust, and cleans it up at least somewhat.

… responses

…ffer()`, or `.json()`

…bodies

…live

# Conflicts:
#	CMakeLists.txt

…n outgoing WASI stream

…bodies

This special-casing used to make sense in the JS-Compute runtime, where certain TransformStreams were more optimizable. It doesn't anymore, and in any case would have to come with actually reading from the stream, which the code as written before this patch didn't 🤷