We all get nullability wrong sometimes

[Richard Andrew Cattermole (Rikki)]

Here is a nice little showcase of what the fast DFA engine is 
able to do:

``class IdentExpr``

https://github.com/higgsjs/Higgs/blob/master/source/parser/ast.d#L986

```d
             catchIdent = cast(IdentExpr)parseExpr(input);
             if (catchIdent is null)
                 throw new ParseError("invalid catch identifier", 
catchIdent.pos);
```

https://github.com/higgsjs/Higgs/blob/master/source/parser/parser.d#L462

Anyone notice a slight problem with this code?

While a sound static analyser would be nice, being able to catch 
code like this does mean bugs don't sit around for 10+ years in 
production code, just waiting to give someone a very bad day.

https://github.com/higgsjs/Higgs/issues/219