Offload source strings from JS heap by using external v8 strings

[AaronO]

This is a specific optimization related to #9660

The problem

Source strings (of runtime JS) take up a decent portion of our baseline-heap, they account for ~1MB of the total ~2.2MB as of 1.8.0, so nearly 50% and is duplicated for each isolate/worker.

The solution

All runtime JS is bundled in the deno binary and available as &'static str so they are inherently immutable, live "forever" and are thus perfect candidates for external strings.

We could simply implement JsRuntime::execute_static, a version of JsRuntime::execute that accepts static filenames and sources and exposes them as external strings to v8.

External strings could possibly be used more broadly, but I think it's best to start with the internal runtime JS.

The impact

We should be able to reduce baseline heap-size by ~1MB or ~50% with little to no downsides (unlike minifying).

The hurdles

This requires extending our rusty_v8 bindings, which is relatively simple (we need to bind to v8::String::NewExternal, expose it twice for one and two-byte strings)

We would also need to ensure that our runtime source files are all truly ASCII (not utf8) if we want to use single-byte strings.

Check if this is compatible with snapshots or what would be required to correct that (from inspecting the heap retainers, all source strings are retained by script_or_debug_info of sub-functions, snapshots without the external strings might result in dangling references that would need to be restored) .

[bartlomieju]

We have already done it, so I'm gonna close this for now.