[enh] implement UDA for module declaration

This PR introduce UDA for module declaration.
Usage:
```
///test.d
@UDA(42) module test;
struct UDA
{
    int a;
}

pragma(msg, __traits(getAttributes, test)); //prints "tuple(UDA(42))"

///test2.d
import test;
pragma(msg, __traits(getAttributes, test)); //prints "tuple(UDA(42))"
```
In other words, when __traits(getAttributes, ...) is applied to import statement, it returns attributes of corresponding module.

Mulitple UDAs are allowed:
```
@UDA(42) @UDA(24) module test; //OK
```
```
@(UDA(42), UDA(24)) module test; //OK
```
UDAs can be mixed with `deprecated` attribute:
```
@UDA(42) deprecated module test; //OK
```
```
deprecated @UDA(42) module test; //OK
```

Why don't you call this in `visit(Module *)`? Then, you can just call `toCBuffer(m, &buf, &hgs)` to pretty-print all module contents (then you can move back `genhdrfile` function position).