[DRAFT] Improve reporting of exception locations

[Andrew Whatson]

Hello guile-dev!

I'm working on a revised patch to improve the reporting of exception
locations, after last month's initial flawed attempt.

The new patch takes the more radical approach of capturing the stack
when an exception is created, including it as part of the compound
exception object.  This should ensure that we capture a correctly
trimmed stack, while avoiding the complexities of pre-unwind handlers.
This is similar to other dynamic languages where it's common to bundle
stack info with exception objects.

This approach will probably have a negative impact on code which is
sensitive to the performance of exception creation; capturing the
stack is more expensive than NOT capturing it.  Is this something that
we need to be concerned about?

I'm still working on integrating these changes into Guile, but thought
it's worth getting early feedback on this one.

Patch to follow!

Cheers,
Andrew