Add grammar.dd to show complete D grammar

Copy `GRAMMAR` part from following pages, tweak grammar, and fix some grammar bugs.

```
attribute.dd
class.dd
declaration.dd
entity.dd
enum.dd
expression.dd
function.dd
iasm.dd
interface.dd
lex.dd
module.dd
pragma.dd
statement.dd
struct.dd
template-mixin.dd
template.dd
traits.dd
unittest.dd
version.dd
```