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. 
Can you really take these out?

We do need to be able to read the AST for debugging purposes, so it needs not to be generating an incorrect AST in the ToString method.