Clean stacktraces for monadic code

[girving]

In monadic code, the functions build computations which are returned and then executed. Once we're actually inside the monad executing code, the original function names are long gone. We need a scheme that produces nice stack traces inside monadic (or other?) code.

[dylex]

Even in non-monadic code, we should eventually make sure that generated (lifted) functions don't show up in stack traces.

[girving]

Well, they should show up, but with correct location information instead of a meaningless manufactured name.