Support if-then-else and case in fixed function body

[lileicc]

related to #198

[cberzan]

This works already:

fixed Real myabs(Real a) =
    if a >= 0 then a
    else -a;

query myabs(123);
query myabs(-456);

fixed Real foo(Integer a) =
    case a in {
        1 -> 100.0,
        2 -> 200.0
    };

query foo(1);
query foo(2);
query foo(3);