Allow designated initialization of struct

[ryuukk_]

Today i tried this:


```D


struct Test
{
     int test;
}

struct Data
{
     Test[8] test;
}

void add(Data data)
{
}

extern(C) void main()
{
     add( Data(test: [ {test: 1}] ) );
}

```

it refuses to compile:

```
onlineapp.d(19): Error: found `}` when expecting `;` following 
expression
onlineapp.d(19):        expression: `1`
onlineapp.d(19): Error: found `]` instead of statement
onlineapp.d(21): Error: found `End of File` when expecting `,`
onlineapp.d(19): Error: found `End of File` when expecting `]`
onlineapp.d(21): Error: found `End of File` when expecting `)`
onlineapp.d(21): Error: found `End of File` when expecting `)`
onlineapp.d(21): Error: found `End of File` when expecting `;` 
following expression
onlineapp.d(19):        expression: `add(Data(test: [()
{
test:
1;
__error__
}
]))`
onlineapp.d(21): Error: matching `}` expected following compound 
statement, not `End of File`
onlineapp.d(18):        unmatched `{`
```


D worse than C with these stupid RAII, give me proper way to 
initialize a struct instead of giving me a broken constructor i'm 
not even using