Insert assignments in JuvixReg to guarantee continuous memory use

[lukaszcz]

Cairo requires memory to be used continuously, e.g., one cannot assign to memory at address 100 and leave memory at address 99 uninitialized at the end of the program (this is checked by the Cairo runner). Hence, we cannot leave any temporary variables in JuvixReg uninitialized. We need to implement a transformation on JuvixReg that initializes variables not assigned in an if/case branch, but assigned in other branches (variables assigned in only one branch will appear e.g. as a result of transforming JuvixReg into SSA form).