JNI: Simplify node context
JNI: Simplify node context

Summary:
@public
Contexts of nodes only hold a bit mask after we got rid of weak JNI refs.
We can simply store that data in a pointer-sized unsigned int.

Here, we replace all context heap allocations with usage of the node context (`void *`) as bitmask. We also add a couple of utility operators in order to keep the code comprehensible.

Reviewed By: fabiomassimo

Differential Revision: D14425742

fbshipit-source-id: f32c2184a1f09268c39dbb8cd09ac96517339674

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