Defect fix: MAGN-3605 and MAGN-3646

Pre-compilation of CBN code no longer involves round-tripping from code to AST and back. Once the code is parsed into AST, it's AST all the way. Existing CBN test case regressions show no failures. A patch made to resolve a defect due to this round-tripping in AssociativeAST.cs was reverted with no side-effects. 
Is this really necessary as a strategy? They are valid expressions, doesn't our parse have an API that we can use to directly parse an expression?