[mypyc] Fix name conflict when creating CPyFunction variable

[p-sawicki]

Local variables for CPyFunction objects are named by taking the C name of the wrapped function and adding _wrapper at the end. This creates a name conflict when a class has two async methods with names X and X_wrapper and means that the CPyFunction variable shadows the function definition.

The function name is used as a function pointer and passing the CPyFunction object instead causes a segfault.

Fix by naming the variables by using a prefix instead. The C function names are already prefixed by CPyPy_ and the class name so there shouldn't be conflicts through Python names.