Fix https://bugs.llvm.org/show_bug.cgi?id=43830 while avoiding polluting the global Python namespace.
This both reverts r357277 to rebundle a version of Python's readline module based on libedit.
However, this patch also provides two improvements over the previous implementation:
- use PyMem_RawMalloc instead of PyMem_Malloc, as expected by PyOS_Readline (prevents to segfault upon exit of interactive session)
- patch the readline module upon embedded interpreter loading, instead of patching it globally, which should prevent any side effect on other modules/packages
- only activate the patched module if libedit is actually linked in lldb