feat(jstz_engine): implement a write-barriered GC pointer abstraction

[johnyob]

Context

Problem: SpiderMonkey has an incremental garbage collector that relies on write barriers to maintain a consistency reachability set.

Intuitive view of incremental GC:

1. Take snapshot of compartment
2. Mark a bit, get interrupted
3. Execute some code
4. Mark a bit again, ...
   ...
5. Compartment fully marked, do sweeping

However, in step 3, the reachability set of objects may change. Write barriers are a solution to notify the GC when the reachability set might change e.g. on a write to a pointer.

Description

This PR implements GcPtr, a safe write barriered pointer.
Dereferencing the pointer is still unsafe since the caller must guarentee that the object that the
pointer points to is valid.

Manually testing the PR

cargo nextest run --package jstz_engine

[codecov]

Codecov Report

Attention: Patch coverage is 58.59031% with 94 lines in your changes missing coverage. Please review.

Project coverage is 46.93%. Comparing base (4fd7190) to head (d874eaf).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/jstz_engine/src/gc/ptr.rs	51.96%	61 Missing ⚠️
crates/jstz_engine/src/script.rs	67.00%	32 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ
crates/jstz_engine/src/lib.rs	100.00% <ø> (ø)
crates/jstz_engine/src/script.rs	23.42% <67.00%> (ø)
crates/jstz_engine/src/gc/ptr.rs	51.96% <51.96%> (ø)

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4fd7190...d874eaf. Read the comment docs.

[zcabter]

Minor updates, LGTM otherwise.