Generate an implicitGeneric base class for new-style generic classes. For this to work, also create C statics that can be used to access type variable objects (e.g.T orTs) at runtime. These are needed when evaluating base classes. ImportTypeVar and friends from the_typing C extension instead oftyping, since the latter is pretty slow to import, and we don't want to add a hidden new runtime dependency in case the fulltyping module isn't needed.

Generic functions don't need any changes, since they don't support indexing with a type, and type variable types aren't valid in runtime contexts. Type erasure seems sufficient, especially considering that mypyc doesn't support classes nested within functions. (I'm not 100% sure about this though, and we might need to put function type variables into statics eventually.)

Update builtins test fixtures used in mypyc tests to not defined type variables such asT, since these leak into tests and can produce unexpected or unrealistic results.

Ignore upper bounds and value restrictions. These are only used for type checking. This should only affect introspection of type variables, which isn't properly supported in compiled code anyway.

New type alias syntax is not supported in this PR.