Multiple underscores are allowed in names for methods

[djavorszky]

According to the Naming Rules in the tutorial:

After an underscore for private or special methods (behaviors, constructors, and functions), any method or variable, including parameters and fields, must start with a lowercase letter. In all cases underscores in a row or at the end of a name are not allowed, but otherwise, any combination of letters and numbers is legal.

I understand this as: multiple consecutive underscores are not allowed (and some other rules). However, it seems to be allowed for parameters (arguments), as the following compiles (and runs) happily:

actor Main
  new create(___: Env) =>
    ___.out.print("Hello world")

  new prime(___': Env) =>
    ___'.out.print("umm....")
    ___'.out.print("Hello prime")

  fun do_someting(___a: Env) =>
    ___a.out.print("Do something")

  be happy(___a_: Env) =>
    ___a_.out.print("Be happy")

Link to playground: https://playground.ponylang.io/?gist=327bddd28fbe66b24d58c681a93d60f2

There are two issues:

1. only underscores are accepted
2. If we're doing an underscore-only prime (___'), the syntax highlighting breaks on the playground (though I guess this isn't an issue per se, as it only occurs because issue1 exists)

Cheers!

[SeanTAllen]

check_id_param isn't being called anymore. The was removed with case functions in #2542. The removal of check_params in sugar.c shouldn't have happened. Other check methods perhaps shouldn't have been removed as well.

Tests should be added for anything that was incorrectly removed so they can be removed again in the future.