[mypyc] Generate function wrappers for each callable class instance

[p-sawicki]

When generating callable classes for async functions, currently mypyc will generate a single CPyFunction wrapper for each callable class type. This results in incorrect behavior when the callable class is used as a wrapper because the @wraps(fn) decorator will set the __name__ attribute of the callable class to name of fn.

If the callable class is instantiated multiple times, the name of previous instances will be overridden because all instances share the same CPyFunction wrapper which contains the name attribute.

To fix this, the CPyFunction wrappers are generated for each instance of the callable class instead of for each type. The wrappers are stored as an attribute of the callable class and instantiated in the setup function.