Adding scoped local set/param declaration to GMPL

Hello !

I'm experimenting with allow local set/param declarations inside scoped 
blocks (for/if/then/else), it's the base to possibly experiment further 
with problem/callback/function declarations you can see/experiment with 
it in this branch https://github.com/mingodad/GLPK/tree/local-set-param 
, look at 
https://github.com/mingodad/GLPK/blob/local-set-param/examples/shikaku-if.mod 
and 
https://github.com/mingodad/GLPK/blob/local-set-param/examples/test-if.mod 
to see it's usage.

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for{i in 1..4} {
     printf "now we are at %d\n", i;
     if i mod 2 = 0 then { #assert when missing "= 0"
         param lp := i+10;
         set ls := {1..i+1};
         printf "nested if lp = %d\n", lp;
         display ls;
     }
     param lp := i+20;
     set ls := {1..i+2};
     printf "nested for lp = %d\n", lp;
     display ls;
}

====

Any comment/suggestion is welcome !

Cheers !