- Made sqlalchemy.sql.expressions.Executable part of public

  - Made sqlalchemy.sql.expressions.Executable part of public
    API, used for any expression construct that can be sent to
    execute().  FunctionElement now inherits Executable so that
    it gains execution_options(), which are also propagated
    to the select() that's generated within execute().
    Executable in turn subclasses _Generative which marks
    any ClauseElement that supports the @_generative
    decorator - these may also become "public" for the benefit
    of the compiler extension at some point.