dillon1000/react

[hir] infer reference effects for property call

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This PR does not add inference for normal `CallExpression`s, since built-in 
functions for `Array` and `Object` are usually only valid if called with a 
correctly-typed `this`. If we want codegen to preserve source code semantics, 
Forget should only add inferred types it is confident about. 

This PR also adds `returnEffect` to FunctionSignature. `returnEffect = Store` if 
this function is known to always return a captured value from `receiver` or 
`args`.