Better Fabric ShadowNode memory safety

[NickGerleman]

Summary:
This fixes a few instances where YogaLayoutableShadowNode (or general shadownode casting) could offer better memory safety.

1. The reference form of traitCast() now terminates on invalid cast, instead of debug assert, since it is better to crash in production than to corrupt memory (which will crash somewhere later, in a much more confusing way).
2. We use traitCast() in more places where we previously would static_cast. This means needing to formally add a mutable version.
3. We bounds-check yoga children access in a single place by using std::vector at() instead of [].
4. Removed Trait::UnreservedTrait1 API, since multiple libraries using it can collide and we lose the memory safety benefits of traitCast.

This change is in response to a bug where YogaLayoutableShadowNode may perform an invalid static_cast of RawTextShadowNode if a text or number is rendered directly inside of a <View> (instead of a <Text> element).

This does not yet fix the underlying logic of YogaLayoutableShadowNode to act gracefully when a RawTextShadowNode makes its way into children. We just terminate, instead of corrupting memory.

Changelog:
[General][Breaking] - Better Fabric ShadowNode Memory Safety (Removes Trait::UnreservedTrait API)

Differential Revision: D43271779

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D43271779

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
android	hermes	arm64-v8a	8,459,768	+1,134
android	hermes	armeabi-v7a	7,783,237	+1,342
android	hermes	x86	8,936,020	+1,485
android	hermes	x86_64	8,793,168	+1,544
android	jsc	arm64-v8a	9,093,922	+1,150
android	jsc	armeabi-v7a	8,292,213	+1,344
android	jsc	x86	9,145,075	+1,485
android	jsc	x86_64	9,403,993	+1,538

Base commit: 9718c17
Branch: main

[facebook-github-bot]

This pull request has been merged in c2a089f.