Distinguish between function expressions and function declarations #489

Currently we only support function declarations in FuzzIL, i.e. something like

function foo() {
    ...
}

But not

let foo = function() {
    ...
}

For the most part, it doesn't really matter whether we emit function statements or expressions, but in certain situations the difference is observable, for example here:

let x = function foo(n) {
  if (n == 0) return;
  foo(n-1);
}
x(10);

I'm not sure how best to support this. Probably the easiest way would be to simply add a boolean flag to BeginAnyFunctionDefinition that states whether the function should become an expression or a statement.