SUPPLIED for generalized variables

[okamsn]

The SUPPLIED variable should not be a generalized variable, but its necessity
can only be determined when parsing the generalized variable. Therefore, could
we must modify loopy--destructure-generalized-sequence to return a list of two
elements, being the normal generalized variables and the normal iteration
variables?

The iteration variables would be evaluated in a setq form at the commands
location in the body, much like the variables from iteration commands.