Type state analysis

[Walter Bright]

The @live functions do type state analysis - data flow analysis is used to 
determine if a variable is 'live' or not.

It is indeed costly to do dfa in the front end, that's one reason why it's 
restricted to @live functions.

The dfa could be extended for null checking, but in practice, null checking is 
not that effective:

```
class C { void xx(); }

struct S { C c; }

C mars(C c) { return null; }

void phobos(ref S s)
{
     C c;
     c.xx();       // detected
     mars(c).xx(); // needs whole program DFA to detect
     s.c.xx();     // cannot be detected
}
```

This is why @live functions won't work without scope, ref, and return 
annotations on the functions it interfaces with. @live functions, like Rust, 
also severely limit the kinds of data structures possible.

Other type state analysis currently done in D is:

1. Value Range Propagation

2. whether a field is initialized or not in a constructor is tracked