Set up bind as an alias for >- in errormonad

This is so interactive uses can be set up to avoid the >- token, which
is handled magically by the quotation filter in "interactive"
settings, and which can thereby cause much annoyance.

Provide a way to use `with_flag' in the errormonad

This ensures that the reference is updated (and restored) when the
monadic-computation is actually run, rather than when the monad is
generated/computed. Of course, if you want to have a reference change
during generation/computation, just use with_flag.

Move some utility monad-related code to portableML