int x = auto

[Ogi]

Suggestion: allow implicit default-initialization using `type x = 
auto` syntax. This is the same as `type x = type.init` and `type 
x = type()`.

This will be especially handy for functions with optional 
parameters.

Before:
```D
void fun(
         int x = 0,
         double x = double.nan,
         bool flag = false,
         string str = "",
         void* ptr = null,
         SomeStruct = SomeStruct(),
         int[4] arr = [0, 0, 0, 0])
```

After:
```D
void fun(
         int x = auto,
         double f = auto,
         bool flag = auto,
         string str = auto,
         void* ptr = auto,
         SomeStruct = auto,
         int[4] arr = auto)
```

Possible alternatives:
* `int x = init`: most obvious, but this would require making 
`init` a reserved keyword.
* `int x = default`: more clear than `auto` but also quite longer.
* `int x = {}`: incompatible with D syntax.