multi if GP language extension

[Bill Allombert <Bill.Allombert@math.u-bordeaux1.fr>]

Dear PARI developpers,

I have added a branch bill-multi_if
which implement the multi-if extension.
if(a,b,c,d) is an alias for if(a,b,if(c,d)),
if(a,b,c,d,e) is an alias for if(a,b,if(c,d,e)),
if(a,b,c,d,e,f) is an alias for if(a,b,if(c,d,if(e,f)),
if(a,b,c,d,e,f,g) is an alias for if(a,b,if(c,d,if(e,f,g)),
etc...

This allow to replace
if(a, b,
   /*else*/ if (c, d, 
      /*else*/ if (d, e,
                 f)))

by
if(a, b,
  /*else if*/, c, d
  /*else if*/, e, f)

This can also be used a case statement of sort:

if(x==0,"0",
   x==1,"1",
   x==2,"2",
   "default")

Please experiment with it.

Cheers,
Bill.