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NLR buffers are usually quite large (use lots of C stack) and expensive to
push and pop.  Some of the time they are only needed to perform clean up if
an exception happens, and then they re-raise the exception.

This commit allows optimizing that scenario by introducing a linked-list of
NLR callbacks that are called automatically when an exception is raised.
They are essentially a light-weight NLR handler that can implement a
"finally" block, i.e. clean-up when an exception is raised, or (by passing
`true` to nlr_pop_jump_callback) when execution leaves the scope.
Signed-off-by: Damien George <damien@micropython.org>