open intervals and type derivation

open intervals and type derivation

When dealing with open intervals, preserving the openness of a bound in
a result depends on the operation being strictly monotonic, rather than
merely monotonic.  However, many operations which at first sight seem
strictly monotonic are in fact not, including:
- squaring (expt least-positive-double-float 2) = (expt 0d0 2)
- coercion (coerce (1+ double-float-epsilon) 'single-float)
  = (coerce 1d0 'single-float)

Modify various coercion and interval functions to include conservatism
in these situations.  (Fixes lp#997528)