Removing global state

[marcoeilers]

Removing global state from Silver in order to be able to run multiple instances of Silicon or Carbon in parallel (which several frontends would like to do).

In particular,

• we split the FastParser object into a stateless object that supplies constants and defines some parsing primitives, and a class FastParser that contains mutable state (e.g., related to the current file and positions).
• The ParserExtensions object (which contains mutable state) is moved into the FastParser class, s.t. each parser instance has its own ParserExtensions object.
• Some fresh-value-counters in ParseAST classes are replaced by AtomicInt objects to allow concurrent accesses.

As a result, all code that used to directly access FastParser is now passed an instance of FastParser instead or has to create one itself. In particular, the interface of Viper plugins is extended s.t. their constructor now gets an additional FastParser argument.

[marcoeilers]

@JonasAlaif So what do I do here now to get the backend tests to pass (since this PR depends on a Silicon PR viperproject/silicon#635 and vice versa)?

[JonasAlaif]

@marcoeilers merge the silicon PR first (also looks like you'll need to update carbon) and afterwards either a) rerun the tests here or b) just merge it without rerunning the tests since you saw that those tests passed in the silicon PR with the same version of silver

[jcp19]

I am not super well-versed in this part of the code, so I would suggest waiting for another review before merging. From my side, I have some minor remarks

[vakaras]

The approach sounds good, but I am unfamiliar with the code to review it properly.