Clean up type-checking in builtin functions to match Python.
Add missing nb$add and nb$inplace_add methods to list, tuple, and string objects.
Call nb$add methods of sequence types when using a + operator.
Make sum handle non-number objects and throw errors like Python.
Add missing nb$multiply and nb$inplace_multiply methods to list, tuple, and string objects.
Call nb$multiply methods of sequence types when using a * operator.
Test case to check user-defined operator overloading methods.
Fix builtin pow function to properly handle nmber and lng objects.