dillon1000/react

Ensure valid mutable ranges for all scopes; fix ranges for context vars

Our current validation fails to detect some invalid cases of mutable ranges — 
namely, ranges that are fully or partially uninitialized, with start or 
start+end still set to zero. 

This PR fixes these cases, starting by validating that the ranges for _all_ 
reactive scopes are valid: start >= 1, and end <= (last instr id + 1). This 
exposed the invalid cases, which are also fixed here: 

* During AnalyzeFunctions, we need to reset identifier ranges and scopes when 
exiting an inner function. This has to happen *after* the effects have been 
translated to the function deps/context operands, in order for 
InferRefenceEffects to continue working on the outer function. Previously I did 
this at the start of InferReactiveScopeVariables, but that's insufficient bc the 
incorrect ranges could also influence InferMutableRanges. AnalyzeFunctions is 
the point at which we compute the ranges for identifiers in the inner function, 
so it's the most ideal place to clean those up so they don't influence the outer 
function. 

* In InferMutableLifetimes, we need to ensure that context variable identifiers 
end up with a mutable range starting where they are declared, and ending with 
their last assignment. We now track declarations and extend their mutable range 
to account for each reassignment.