feat(ext/ffi): Make External pointers keep reference to V8 buffer

[aapoalas]

Previously when pointers were simply numbers, there was no way for a pointer number created from a buffer to keep the original buffer alive. This risked Deno.UnsafePointer.of() created pointers becoming dangling if the user did not manually make sure to save the original buffer somewhere.

I have personally run into this previously, to disastrous (segfaulty) results. Based on this I felt the need to explicitly warn of this issue on the Denonomicon: https://denonomicon.deno.dev/types/pointers#dangling-pointers.

Now that pointers are objects themselves, we can get rid of this issue by using a WeakMap to hold the of() buffer parameter with the pointer object acting as the key. This ensures that the buffer stays alive as long as the pointer object does, while holding a weak reference to the pointer object itself to avoid a memory leak.

[littledivy]

@aapoalas What is the performance impact?

[aapoalas]

From about 100ns to 300ns, IIRC. But this is not a particularly hot code path generally.

[littledivy]

LGTM! IMO Fair perf tradeoff since this is rarely used in a hot path.