A call new_constant(n,ty) installs a new constant named n in
the current theory. Note that new_constant does not specify a value
for the constant, just a name and type. The constant may have a
polymorphic type, which can be used in arbitrary instantiations.
FAILURE
Never fails, but issues a warning if the name is not a valid constant
name. It will overwrite an existing constant with the same name in
the current theory.