Auto merge of #20314 - Xanewok:remove-heap-constructor, r=jdm

Don't use unsafe Heap::new constructor

<!-- Please describe your changes on the following line: -->
Pulls https://github.com/servo/rust-mozjs/pull/398 and aims to close https://github.com/servo/rust-mozjs/issues/343.

We can't convert from `JSVal` to `Heap<JSVal>` safely (due to GC barriers we can't move Heap value after changing its underlying value to something meaningful, e.g. non-null or non-undefined), so I decided to also wrap the Heap values in a Box (and in dictionaries in RootedTraceableBox, see https://github.com/servo/servo/pull/20265#issuecomment-372838379 and the issue for more details) in dictionaries.

Since we allocate more to be safe, I think it'd be good to also do some sort of a JS perf run, if there is any to see if there's any significant overhead.

r? @jdm

---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because checking for not moving Heap after setting a value would require encoding a lot more info in type system (Heap) and I'm not sure how to do that and end up with an ergonomic and consistent API

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Cargo.lock

@@ -1617,7 +1617,7 @@ dependencies = [
  "cssparser 0.23.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "euclid 0.17.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "hashglobe 0.1.0",
- "mozjs 0.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "mozjs 0.3.0 (git+https://github.com/Xanewok/rust-mozjs?branch=remove-heap-constructor)",
  "selectors 0.19.0",
  "servo_arc 0.1.1",
  "smallbitvec 1.0.6 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -1792,8 +1792,8 @@ dependencies = [
 
 [[package]]
 name = "mozjs"
-version = "0.2.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
+version = "0.3.0"
+source = "git+https://github.com/Xanewok/rust-mozjs?branch=remove-heap-constructor#d8b6ae7fcff342c2fddca8df5a7556bcc3b9e415"
 dependencies = [
  "cmake 0.1.29 (registry+https://github.com/rust-lang/crates.io-index)",
  "lazy_static 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -2467,7 +2467,7 @@ dependencies = [
  "mime_guess 1.8.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "mitochondria 1.1.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "mozangle 0.1.6 (registry+https://github.com/rust-lang/crates.io-index)",
- "mozjs 0.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "mozjs 0.3.0 (git+https://github.com/Xanewok/rust-mozjs?branch=remove-heap-constructor)",
  "msg 0.0.1",
  "net_traits 0.0.1",
  "num-traits 0.1.37 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -3862,7 +3862,7 @@ dependencies = [
 "checksum miow 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)" = "8c1f2f3b1cf331de6896aabf6e9d55dca90356cc9960cca7eaaf408a355ae919"
 "checksum mitochondria 1.1.2 (registry+https://github.com/rust-lang/crates.io-index)" = "9de3eca27871df31c33b807f834b94ef7d000956f57aa25c5aed9c5f0aae8f6f"
 "checksum mozangle 0.1.6 (registry+https://github.com/rust-lang/crates.io-index)" = "1f0583e6792917f498bb3a7440f777a59353102063445ab7f5e9d1dc4ed593aa"
-"checksum mozjs 0.2.0 (registry+https://github.com/rust-lang/crates.io-index)" = "79645a302ffcef7ca2673ff4db4d4273b87671dd0de4d53da889b664dd555e74"
+"checksum mozjs 0.3.0 (git+https://github.com/Xanewok/rust-mozjs?branch=remove-heap-constructor)" = "<none>"
 "checksum mozjs_sys 0.50.1 (registry+https://github.com/rust-lang/crates.io-index)" = "e61a792a125b1364c5ec50255ed8343ce02dc56098f8868dd209d472c8de006a"
 "checksum mp3-metadata 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)" = "4ab5f1d2693586420208d1200ce5a51cd44726f055b635176188137aff42c7de"
 "checksum mp4parse 0.9.1 (registry+https://github.com/rust-lang/crates.io-index)" = "f821e3799bc0fd16d9b861fb02fa7ee1b5fba29f45ad591dade105c48ca9a1a0"