Decoupling parser structure from the main VM structure.

Decoupling parser structure from the main VM structure.

vm->parser is only needed during parsing phase, so it can be eliminated.
This improves dependencies tracking and readability.

As a side effect it fixes #372 issue on Github: previously, Function
constructor left VM context in inconsistent state if compilation failed.
The direct root cause was that a function object was created, but
because Function constructor ended prematurely the object was not fully
initialized. This is not a problem by itself because usually this
partially created object cannot be referenced. In the accumulative mode,
which is only enabled in CLI, vm->parser was used to store the references
to the variables from the previous iteration.