Further cleanups for type coercion: treat the locution typename(argument)

Further cleanups for type coercion: treat the locution typename(argument)
as representing a type coercion request in more cases than we did before.
It will work now whenever no underlying function is required, ie if the
coercion is binary-compatible or if the argument is a previously untyped
string constant.  Otherwise, you still need a real function to exist.