fix: constraint generation with duplicate variables

[DavePearce]

Summary

Constraint generation fails when there are multiple variables of the same name in the same function. This can happen in ZkC due to block scoping.

Example 1

Consider the following program:

fn f(x:u16){
   if x == 0 {
      var i:u16 = x+1
   } else {
      var i:u16 = x+2
   }
}

This generates the following constraints (which, as far as I am aware are correct, but could cause problems in e.g. the prover):

(module f)

(inputs
   (x u16 0x0)
)

(computed
   (i u16 0x0)
   (i u16 0x0)
   ($3 u0 0x0)
)


(vanish f:pc0
   (∧ (== $3 0)
      (if (== x $3)
         (== i (+ 1 x)))
      (if (!= x $3)
         (== i (+ 2 x)))))

Example 2

The following fails when generating AIR constraints specifically:

fn f(x:u16) -> (r:u16) {
    r = x
    //
    for i:u8 = 0; i<5; i = i + 1 {
        r = r + 1
    }
    //
    for i:u8 = 0; i<3; i = i + 1 {
        r = r + 2
    }
    //
}

This results in a panic: register "i" already declared error.