Allocate exception/its info lazily

[pfalcon]

Currently, uPy allocates exception and its attribute, like backtrace, eagerly. Might postpone e.g. traceback allocation until later, and store basic exception info statically. E.g., if exception is caught by except clause without as, then the actual exception object is not needed, only its type is needed.

[pfalcon]

This is apparently possible only with stackless build, because it's the case where we have real chain of function frame records which we can walk (instead of capturing them as backtraces, as we unwind real C stack on which they may be stored.