SSA generation and register allocation update.

[Milky2018]

Main contributions:

• optimized_option type for constructor expressions are now supported. See 848a471 and 666cd17.
• Register allocation update: now loops with multiple back edges are supported. See 365893f.
• Support loops that return a value as well as tail-recursive functions (which will be optimized to loops). See 09e191f. Examples are list01, list02, recursive01.

Test cases:

• User-defined list types 122d6bf
• Tail recursive functions 40c6a69

SSA interpreter:

• Now middle output will not be removed so we can easily check them. 650c846