For some reason assertions are disabled when running mypyc tests. Explicitly undefining NDEBUG when compiling enables them again so I have added compiler options to undefine it when requested optimization level is 0.

Found two issues after enabling them:

• When interning strings there is an assertion that makes calling the function multiple times crash. It might be called multiple times when multiple modules are initialized within one compilation unit so I have added an early return when the strings are already interned.
• Py_SIZE has an assertion to disallow calling it with PyLong arguments starting from python 3.12. Compiled code would call it when converting PyLong to int64. Instead we can use PyUnstable_Long_CompactValue.