bpo-40334: refactor and cleanup for the PEG generators

[pablogsal]

Improvements in this PR:

• No more relying on the string names to deduce the types of local variables.
• No more threading down the names of local variables from all functions and
  visiting: now the code has a local variable scope with a context manager.
• Specially for @lysnikolaou: I have actually managed to do bpo-40334: Refactor peg_generator to receive a Tokens file when building c code #19745 (comment) and now we don't need all the specialized token functions. Just one for name, string and number.
• Now we have a class to represent function calls, making the code a bit
  more explicit and carrying metadata if needed. Also helps when debugging (from experience 😉 ).
• Fix some mypy errors (like the fact that in add_var the variable type could actually have been None as opposed to our previous declaration).

https://bugs.python.org/issue40334

[lysnikolaou]

• Specially for @lysnikolaou: I have actually managed to do #19745 (comment) and now we don't need all the specialized token functions. Just one for name, string and number.

😃 😃

[lysnikolaou]

This is really a great improvement! Thanks a a lot! Most of my comments are stylistic, so feel free to ignore them.

[lysnikolaou]

Yes! 🚀

[pablogsal]

The buildbot failures are unrelated and they are being addressed here:

#19791

[gvanrossum]

Thanks for this one too!