Switch exceptions used for control flow from global to local

[jberdine]

Now that ocaml 4.04 can be relied on, the exceptions used for control flow
can be switched from global to local. The motivation is to avoid nested
operations from catching exceptions not intended for them, such as a
fold_while whose f argument internally also performs a fold_while.

Signed-off-by: Josh Berdine [email protected]

[c-cube]

thanks!