feat(jstz_engine): define the Trace and Finalize traits

[johnyob]

Context

We'll need to trace native Rust objects. To do so, we'll need a Trace trait (and a Finalize for drops)

Description

This PR:

• Defines the Trace and Finalize traits + some helpful macros
• Implements the Trace and Finalize trait for many common types.

Manually testing the PR

Code isn't easily testable (relies on calling out to SM GC where Tracer is defined). I suggest we ignore the codecov check for this PR -- these implementations will become tested as we implement APIs on top of them

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 185 lines in your changes missing coverage. Please review.

Project coverage is 45.30%. Comparing base (96d3d75) to head (f55488f).

Files with missing lines	Patch %	Lines
crates/jstz_engine/src/gc/trace.rs	0.00%	182 Missing ⚠️
crates/jstz_engine/src/gc/ptr.rs	0.00%	3 Missing ⚠️

Files with missing lines	Coverage Δ
crates/jstz_engine/src/gc/ptr.rs	50.76% <0.00%> (-1.20%)	⬇️
crates/jstz_engine/src/gc/trace.rs	4.67% <0.00%> (ø)

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