Examples of DFA

[Walter Bright]

This is what I am talking about:
```
@nogc void foo() { bar(); }

void bar() { abc(); }

void abc() { bar(); }
```
Both bar() and abc() are @nogc. But the forward-only checking cannot deduce it, 
and the result is:

test.d(3): Error: `@nogc` function `test.foo` cannot call non- at nogc function 
`test.bar`

This is the simplest version of this problem. It can be quite complex, but the 
result is the same - it cannot be figured out without solving recursive data 
flow equations.

Another version of the problem:
```
@nogc void foo() { bar(); }

void bar() { abc(); }

void abc();
```
The body of abc() is not available to the compiler, so the compiler cannot 
deduce @nogc for abc(). So, in order to deduce attributes of abc(), the compiler 
must have the source code available for it. To do a complete job, then, requires 
compilation of the entire program's code base. Which will require a lot of 
memory and time.

Adam, Rikki, if I'm wrong, please show me how.