Control flow analysis

[0xd34d10cc]

There is a problem in #15, following program:

fun lol() { return 42 } // function
fun kek() {}            // procedure

lol();
write(kek())

Fails to execute in Statements language (call to procedure kek in expression), but runs successfully in SM (writes 42 to output). Fix: add returns_value flag to Function and check that every Call in Expr .