dillon1000/react

ReactiveFunctionTransform allows replacing values w transformValue

Previously the only way to replace a value was to override transformInstruction 
and transformTerminal, and to be careful to find nested values. This PR adds 
`ReactiveFunctionTransform#transformValue()` which allows returning an optional 
new value, which if present will replace the value in whatever context it 
appeared. ReactiveFunctionTransform now reimplements all the methods necessary 
to replace any value anywhere in the AST. See the next PR for an example usage.