Fix let frame optimization

[joe-p]

foo(a: uint64) {
  assert(a[0])
  let b = a
  b = [1,2,3]
}

Will generate the following TEAL, which retroactively overwrites the value of the arg. I think this should be fixed when #176 is fixed.

        // tests/contracts/general.algo.ts:102
	// assert(a[0])
	byte 0x000000000000000100000000000000020000000000000003 // a: uint64[]
	store 0 // full array
	load 0 // full array
	extract 0 8
	btoi
	assert

	// tests/contracts/general.algo.ts:105
	// b = [1, 2, 3]
	byte 0x000000000000000100000000000000020000000000000003
	frame_bury -1 // b: uint64[] HERE
	retsub